I see:

can't access property "browserPageAction", pageActionAPI is undefined pageActionExtras.js:35
    setLabelForHistogram jar:file:///Applications/Firefox Nightly.app/Contents/Resources/browser/features/webcompat-reporter@mozilla.org.xpi!/experimentalAPIs/pageActionExtras.js:35
    setLabelForHistogram self-hosted:1173
    result resource://gre/modules/ExtensionParent.jsm:956
    withPendingBrowser resource://gre/modules/ExtensionParent.jsm:489
    result resource://gre/modules/ExtensionParent.jsm:956
    callAndLog resource://gre/modules/ExtensionParent.jsm:918
    recvAPICall resource://gre/modules/ExtensionParent.jsm:955
    AsyncFunctionNext self-hosted:690

and

can't access property "action", pageActionAPI is undefined pageActionExtras.js:25
    setDefaultTitle jar:file:///Applications/Firefox Nightly.app/Contents/Resources/browser/features/webcompat-reporter@mozilla.org.xpi!/experimentalAPIs/pageActionExtras.js:25
    setDefaultTitle self-hosted:1173
    result resource://gre/modules/ExtensionParent.jsm:956
    withPendingBrowser resource://gre/modules/ExtensionParent.jsm:489
    result resource://gre/modules/ExtensionParent.jsm:956
    callAndLog resource://gre/modules/ExtensionParent.jsm:918
    recvAPICall resource://gre/modules/ExtensionParent.jsm:955
    AsyncFunctionNext self-hosted:690

and I also saw one referencing helpMenu but I can't get that to reproduce right now.

I see the messages referenced in comment #0 on a clean profile both on windows and macOS, in today's and yesterday's nightlies.

Regression range: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=11c435c7c42f2ace2d9cc977d67248b23e8adb22&tochange=ef36e9cc55b9e56aa643c1e64f1f37dcf5f6901b

Looks like bug 1690573. Mike, can you take a look?

This patch does remove the pageActionExtras experimental API, which is what started triggering
those errors after the pageAction manifest property was removed from the manifest in Bug 1690573:

• The pageActionExtras API was internally retrieving the pageAction API class
  (see https://searchfox.org/mozilla-central/rev/eeb8cf278192d68b3977d0adb4d43f1463439269/browser/extensions/report-site-issue/experimentalAPIs/pageActionExtras.js#12-16)
  which is a bit of an hack and it used to work because there was a pageAction property in the
  manifest.json file (and that was what used to guarantee that the ext-pageAction.js API module
  was always loaded before the background page of the extension did have a chance to run)

• In Bug 1690573 we removed the pageAction property from the manifest and as a side-effect the
  ext-pageAction.js module wouldn't be loaded anymore, which did break the assumption in
  the pageActionExtras API (and the extension background page script as a side effect of that)

I assume that if we removed the pageAction from the manifest, then we don't need the pageActionExtras
experimental API neither (it seems that its purpose was to localize the pageAction and to set a custom
telemetry label for it) and so to fix the regression I opted to remove the remaining traces of the
pageActionExtras experimental API.

NOTE: there are some more traces leaft in head.js, which are used for the test cases.
All the tests are currently disabled and it seems that they are meant to be re-enabled
as part or as a follow up of Bug 1693119 and so I think that for those parts we can
wait to get there.

(In reply to :Gijs (he/him) from comment #2)

Regression range: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=11c435c7c42f2ace2d9cc977d67248b23e8adb22&tochange=ef36e9cc55b9e56aa643c1e64f1f37dcf5f6901b

Hi Gijs,
Thanks a lot for reporting this issue and looking for a regression range.
From a quick look to the regressing change I was able to see what was actually going on, in comment 3 there is a patch with a proposed fix (and a more detailed description of the reasons behind the issue).

Looks like bug 1690573. Mike, can you take a look?

Hey Mike,
given that I was able to see what was going on based on a quick look to the regressing change I'm clearing your needinfo and I added you as an optional reviewer on the attached patch.

https://hg.mozilla.org/mozilla-central/rev/b6227a5fd54a

pageActionExtras.js is still referenced in .eslintrc despite its removal here. It should be removed: https://searchfox.org/mozilla-central/rev/9043e515e9608cc55b252a40cb2dfb6f767bcffd/.eslintrc.js#547

(In reply to Rob Wu [:robwu] from comment #7)

pageActionExtras.js is still referenced in .eslintrc despite its removal here. It should be removed: https://searchfox.org/mozilla-central/rev/9043e515e9608cc55b252a40cb2dfb6f767bcffd/.eslintrc.js#547

Hey Rob,
thanks for pointing it out, could you file it separately? it is not a big deal, but I doubt that we may recall to do that any time soon if the only trace about that is a comment in a closed bug.