Affected versions

• 87.0a1 (20210209214921)
• 86.0b8 (20210209185733)
• 85.0.2 (20210208133944)

Affected platforms

• Windows 10x64
• Ubuntu 18.04
• macOS 10.15

Steps to reproduce

1. Open Firefox with -migration -p command.
2. Select to import data from any browser.
3. After importing is complete, open the browser console.

Expected result

• No errors are displayed.

Actual result

• Several
  can't access property "loadFrameScript", browser.messageManager is undefined - ExtensionParent.jsm:430 this.browser.loadURI is not a function - ext-backgroundPage.js:70 errors are displayed in the browser console.

Regression range

• I will search for one ASAP.

Notes

• The errors are not displayed if the data is imported from Library -> Import data and Backup -> Import Data from another browser.
• Attached a screen recording.
• Attached console log: link.

Hello! I have managed to find a regression for this issue, here is the pushlog:

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=268543e53e1b11ce0e468d985ea3777563e7b8a8&tochange=a7a6063bd5a4b49911b5a956c252a16961aec5a0

This issue does not appear to be specific to extensions. There is also the following error message before any extension errors:

TypeError: can't access property "addMessageListener", this.mm is undefined SanityTest.jsm:294:7

references code at https://searchfox.org/mozilla-central/rev/8d9564059dbc1e36fb9152e6b6227343d0b49662/toolkit/components/gfx/SanityTest.jsm#291,297

Based on the analysis below, it's more likely an issue with timing related to migration, and/or registration of the custom element (browser-custom-element.js).

Questions for reporter:

• Is the console output from the report consistent?
• Do these errors always appear, and in that order?
• Is anyone looking into the SanityTest.jsm errors? If this is consistently the first error in the log, it may be easier to investigate than to investigate the more complicated flow through the implementation of the extension framework.
• When this error occurs, does the Screenshots add-on work? (probably not)

Both SanityTest.jsm and the extension code (ExtensionParent.jsm / ext-backgroundPage.js) have a similar error, that the <browser>'s get messageManager method have returned undefined: https://searchfox.org/mozilla-central/rev/8d9564059dbc1e36fb9152e6b6227343d0b49662/toolkit/content/widgets/browser-custom-element.js#471,477 (OR the MozBrowser custom element in browser-custom-element.js is not registered, in which case browser.messageManager doesn't trigger the getter and the undefined value is seen).

The extension code only reaches that point at https://searchfox.org/mozilla-central/rev/8d9564059dbc1e36fb9152e6b6227343d0b49662/toolkit/components/extensions/parent/ext-backgroundPage.js#59,67,70
when XULFrameLoaderCreated has been observed at https://searchfox.org/mozilla-central/rev/8d9564059dbc1e36fb9152e6b6227343d0b49662/toolkit/components/extensions/ExtensionParent.jsm#1177,1199,1208-1209
... so the frameLoader should be there, yet somehow browser.messageManager and/or (browser.)frameLoader.messageManager is undefined