Latest Firefox Developer Edition Rearranges Bootstrap Responsive Grid Layout

Hi all, I just yesterday upgraded my version of Firefox Developer Edition to 86.0 Beta 6 from 86.0 Beta 2 and for some reason the layouts of my web application are all shifted around. This is occurring in applications using both Bootstrap 3 and 4. Everything is fine in Firefox (standard) version 85.0.1 (and any other browser). Am I missing something with this new version? Did I flip a switch that wasn’t meant to be flipped? Am I in the wrong group to be bringing this up? Thank you!

If you have some time, do you want to run a regression to see whether you can find the check-in that caused it?

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Okay, narrowed it down via this regression utility. The attached image shows the process, but it was narrowed down to this diff version:

Commit Message:

Bug 1686310 part 7 - Remove ReflowInput::mFrameType and associated code, which is now dead code. r=emilio

Honestly I didn’t expect it to go back that far. According to my version/download history via Preferences in my current install of Dev Edition, my last download was on Jan 28th, and I’m certain it was working fine then. So that has me a little baffled.

Regardless, it’s pretty clear the referenced diff is when things took a turn for the worse.

Bug 1686310 seems to imply that some breakage is expected as a result of this change and will need to be addressed individually: “Fortunately, it’s not used very much so it’s easy to surface the underlying conditions directly where they are used.”

So maybe file a new bug?