[Tracking Requested - why for this release]:
Regression introduced in 78, crash with external software.

Steps:

1. Download the Fusion Public Beta from this page and install it.
2. After restarting your computer, start Fusion first (is normally started automatically the first time after an install), and then Firefox. This order is important.
3. Open the attached test case. This was sent to us by an engineer at Vispero, and I was able to reproduce the crash with it.
4. Tab to the first input box and start typing.
   • Result: A parent process crash with reports like this will be submitted.

The people at Vispero were seeing this crash with Firefox 80, I was able to reproduce this with the 83 Nightly build as well.

This is another regression from bug 1640553.

See steps above.

Marco, does this try build (only just kicked off) help? (I haven't tested myself; this is a speculative fix.)

Unfortunately, this build still crashes with Fusion and the test case. If it is of any use: here is the report for that crash.

Try build, attempt 2

The second try build fixes the crash for me.

https://hg.mozilla.org/mozilla-central/rev/f2e684839464

The patch landed in nightly and beta is affected.
:Jamie, is this bug important enough to require an uplift?
If not please set status_beta to wontfix.

For more information, please visit auto_nag documentation.

Verified in Nightly 83.0a1 20200929213959.

Comment on attachment 9177517 [details]
Bug 1666720: AccessibleHandler: When refreshing the cache, fetch into a temporary struct rather than into the live cache to avoid problems triggered by COM re-entry.

Beta/Release Uplift Approval Request

• User impact if declined: Crashes with ZoomText or ZoomText Fusion 2021 assistive technology products (currently in public beta).
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Only affects Windows accessibility. Straightforward patch which updates a complete cache in two stages instead of one.
• String changes made/needed: None.

Comment on attachment 9177517 [details]
Bug 1666720: AccessibleHandler: When refreshing the cache, fetch into a temporary struct rather than into the live cache to avoid problems triggered by COM re-entry.

a11y crash fix, approved for 82.0b6

https://hg.mozilla.org/releases/mozilla-beta/rev/8de8bc506f25

Do we want to fix this in esr78 also?

Comment on attachment 9177517 [details]
Bug 1666720: AccessibleHandler: When refreshing the cache, fetch into a temporary struct rather than into the live cache to avoid problems triggered by COM re-entry.

ESR Uplift Approval Request

• If this is not a sec:{high,crit} bug, please state case for ESR consideration: Crashes.
• User impact if declined: Crashes with ZoomText or ZoomText Fusion 2021 assistive technology products (currently in public beta).
• Fix Landed on Version: 82, 83
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Only affects Windows accessibility. Straightforward patch which updates a complete cache in two stages instead of one.
• String or UUID changes made by this patch: None.

Comment on attachment 9177517 [details]
Bug 1666720: AccessibleHandler: When refreshing the cache, fetch into a temporary struct rather than into the live cache to avoid problems triggered by COM re-entry.

approved for 78.5esr

https://hg.mozilla.org/releases/mozilla-esr78/rev/3ce9dba91217