Andy McKay

How many Chrome Extensions run in Firefox?

Update: I did a bit more filtering and checking and the numbers dropped to around 76%. Head down past the caveats for more.

This question gets asked quite a lot around Mozilla now that we’ve been working on WebExtensions for a while. We’ve been aiming for some sort of Chrome parity. A while back I wrote a script that I used on arewewebextensionsyet.com. That produced a percentage of WebExtensions, but I always felt nervous about talking about it because there so many caveats it never felt accurate.

One of the major caveats was that the sample size of extensions I had from the Chrome store of 10,000 felt too small (that sample is the one used on arewewebextensionsyet.com currently). This week I used this project to parse the sitemap on the Chrome store and get 100,000 extensions and apps. A much more satisfying amount.

After stripping out apps, I then ran some scripts over those extensions to see how many would work in Firefox Nightly. Here’s the output:

And here is an explanation of the long list of caveats and why I still am very cautious about this number.

Caveats

• This doesn't mean they can run out of the box, it means they are </b>easily convertable</b>. Possibly the most obvious reason is that some of these extensions contain deprecated methods. Firefox hasn't implemented those deprecated methods. But if the developer updates their extension to use the new methods it will work in both Firefox and Chrome.
• This method filters out apps, or extensions which use apps API, which can be a little heavy handed and hard for extension users to spot.
• This method just looks at APIs and permissions. Which means that features that are required beyond those two methods are not covered. Web APIs for example aren't tested.
• Whilst we've got many APIs, some of them may not be implemented exactly the same as Chrome. Or have bugs. That can cause some extensions to have some problems.
• A grep of chrome.* for APIs reveals domains, typos, comments and some just wierd strings. If you ignore those there would probably be even more that work.
• This uses data from Firefox Nightly, if you use earlier versions of Firefox, you'll get less. Also not every single API is ready for release yet.

The result of these caveats is a list of variables which you can see in the code. A more perfect scenario would be to download the extension, try loading it into Firefox and then seeing the result. But automating and building it all out would take more than one hour or two of hacking. If anyone wants to try to do that for 100,000 extensions let me know.

So what can you take from the number 87.76%? Some comfort that its probably somewhere around that number. I can say with a lot of confidence that over 75% of extensions are easily convertable to Firefox. A bit less confidence that over 85% are.

That sounds pretty good to me though and a credit to the hard work the team has put in this year.

Update: November 22nd

A couple of more issues or caveats were pointed out to me:

• This list of add-ons includes themes, those are possible in Firefox already and although its related slightly different. Because themes don’t use APIs and are different they should probably be excluded. By the way, there is a plan to get new theming support in Firefox. This dropped our total number of extensions examined down.
• This does not include manifest keys. Some elements of the manifest do not need an API or permission so if we don’t support them we still aren’t at parity. All the big ones, we already have bugs and plans for, but just to be complete I included these.

The result is now:

Told you I felt pretty good about 75% of extensions. For a more detailed output, check out the repository.

Full output of the script from today:

Importer stats
--------------
  71551 74.81% success
  24041 25.13% apps
     56 0.06% error

   5463 7.64% missing_permissions
   6542 9.14% missing_apis

  62792 87.76% easy_conversion

Missing APIs
------------
   1369 chrome.desktopCapture.chooseDesktopMedia
    533 chrome.permissions.request
    458 chrome.declarativeContent.onPageChanged
    454 chrome.declarativeContent.PageStateMatcher
    443 chrome.declarativeContent.ShowPageAction
    427 chrome.devtools.panels
    360 chrome.identity.getAuthToken
    346 chrome.proxy.settings
    311 chrome.permissions.remove
    292 chrome.devtools.inspectedWindow
    291 chrome.desktopCapture.cancelChooseDesktopMedia
    272 chrome.permissions.contains
    221 chrome.management.setEnabled
    187 chrome.runtime.error
    185 chrome.identity.launchWebAuthFlow
    182 chrome.management.onInstalled
    168 chrome.tts.speak
    167 chrome.identity.removeCachedAuthToken
    146 chrome.browsingData.remove
    141 chrome.tabs.cr_tabs_TabClosed_listeners_added
    141 chrome.tabs.cr_tabs_TabCreated_listeners_added
    141 chrome.tabs.cr_tabs_TabSelectionChanged_listeners_added
    141 chrome.tabs.hasOwnProperty
    128 chrome.identity.getProfileUserInfo
    126 chrome.management.onUninstalled
    126 chrome.devtools.network
    116 chrome.management.onEnabled
    109 chrome.manifest.content_scripts
    107 chrome.extension.onMessageExternal
     99 chrome.management.uninstall
     94 chrome.downloads.onDeterminingFilename
     94 chrome.management.onDisabled
     92 chrome.extension.onClicked
     78 chrome.experimental.cookies
     76 chrome.proxy.onProxyError
     76 chrome.identity.getRedirectURL
     71 chrome.storage.managed
     71 chrome.tts.stop
     61 chrome.privacy.services
     59 chrome.manifest.version
     57 chrome.privacy.network
     56 chrome.tabCapture.capture
     54 chrome.pageCapture.saveAsMHTML
     51 chrome.permissions.onAdded
     50 chrome.tts.getVoices
     46 chrome.browsingData.removeCache
     43 chrome.d.ts
     43 chrome.debugger.attach
     43 chrome.storage.set
     42 chrome.debugger.sendCommand
     40 chrome.permissions.onRemoved
     38 chrome.extension.connectNative
     38 chrome.permissions.getAll
     37 chrome.debugger.onEvent
     37 chrome.debugger.detach
     34 chrome.privacy.websites
     33 chrome.i18n._getL10nData
     33 chrome.i18n._setL10nData
     31 chrome.identity.onSignInChanged
     31 chrome.webRequest.MAX_HANDLER_BEHAVIOR_CHANGED_CALLS_PER_10_MINUTES
     30 chrome.tts.isSpeaking
     26 chrome.fontSettings.getFontList
     25 chrome.storage.get
     24 chrome.contentSettings.javascript
     24 chrome.debugger.onDetach
     22 chrome.webstore.install
     21 chrome.cast.media
     21 chrome.input.ime
     21 chrome.contentSettings.popups
     20 chrome.system.cpu
     20 chrome.cast.SessionRequest
     20 chrome.runtime.onUninstalled
     20 chrome.declarativeWebRequest.onRequest
     20 chrome.cast.ApiConfig
     19 chrome.cast.requestSession
     19 chrome.browsingData.removeHistory
     19 chrome.system.display
     18 chrome.contentSettings.cookies
     18 chrome.cast.initialize
     18 chrome.browsingData.removeDownloads
     18 chrome.browsingData.removeCookies
     17 chrome.tabs.TAB_ID_NONE
     17 chrome.declarativeWebRequest.RequestMatcher
     17 chrome.webRequest.ResourceType
     16 chrome.browserAction.show
     16 chrome.settings.uuuid
     16 chrome.extension.getVersion
     15 chrome.system.storage
     15 chrome.tabs.executeScriptAsync
     15 chrome.contextMenus.ACTION_MENU_TOP_LEVEL_LIMIT
     15 chrome.privacy.IPHandlingPolicy
     15 chrome.runtime.lasterror
     15 chrome.tabs.executeScriptInFrame
     14 chrome.example.echo
     14 chrome.settings.get
     14 chrome.runtime.Port
     14 chrome.contentSettings.images
     14 chrome.extension.onConnectExternal
     13 chrome.ttsEngine.onStop
     13 chrome.settings.set

Missing permissions
-------------------
   1382 desktopCapture
    620 clipboardRead
    616 declarativeContent
    510 proxy
    476 geolocation
    457 browsingData
    348 contentSettings
    275 tts
    232 system.cpu
    158 privacy
    156 debugger
    151 pageCapture
    144 identity.email
    120 tabCapture
     91 sessions
     79 downloads.shelf
     54 fontSettings
     52 unlimited_storage
     39 ttsEngine
     36 system.storage
     32 input
     30 fileBrowserHandler
     29 system.display
     26 declarativeWebRequest
     18 system.memory
     14 processes
     11 power
      8 audioCapture
      7
      7 {u'socket': [u'tcp-connect', u'tcp-listen']}
      7 videoCapture
      7 printerProvider
      6 location
      5 webview
      5 fileSystem
      5 enterprise.platformKeys
      4 create
      4 experimental
      4 infobars
      4 {u'fileSystem': [u'write']}
      4 signedInDevices
      4 wallpaper
      3 networkingPrivate
      3 dial
      3 cast.streaming
      3 app
      3 mdns
      3 cast
      3 streamsPrivate
      3 keybinding
      2 bookmarkManagerPrivate
      2 certificateProvider
      2 preferencesPrivate
      2 activetabs
      2 enterprise.deviceAttributes
      2 {u'fileSystem': [u'write', u'retainEntries', u'directory']}
      2 usb
      2 *
      2 dns
      2 tab
      2 audio
      1 *//yousonnik.ru/*
      1 all_frames
      1 webR/equestBlocking
      1 offroad
      1 hid
      1 @ant-sites-string-list@
      1 newtab
      1 accessibilityFeatures.read
      1 activeTab1
      1 t
      1 .entireweb.com
      1 accessibilityPrivate
      1 externally_connectable
      1 brailleDisplayPrivate
      1 alwaysOnTopWindows
      1 Storage
      1 mangement
      1 clipboardWrite
      1 https:ajax.googleapis.com/
      1 *.facebook.com/*
      1 dataReductionProxy
      1 desktopCapturePrivate
      1 enterprise.platformKeysPrivate
      1 popup
      1 copresence
      1 accessibilityFeatures.modify
      1 platformKeys
      1 beamrise
      1 Tabs
      1 http//gosnapshot.com/snapshot
      1 s
      1 urls
      1 https: //*/*
      1 commandLinePrivate
      1 system
      1 sienium
      1 virtualKeyboardPrivate
      1 www.bgu.ac.il
      1 none