Is this a fairly new issue, for example, since the 139.0b1 release on April 28 or later?

I don't think it's related to the dev tools shortcuts (https://firefox-source-docs.mozilla.org/devtools-user/keyboard_shortcuts/index.html), unless there are undocumented ones.

On my test page, do you still see both keydown and keyup events reported for Ctrl+Alt+[key] and Ctrl+Alt+Shift+[key]:

https://www.jeffersonscher.com/res/keyvents.html

Any difference between left and right Alt keys?

Is anything customized if you check here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste ui.key and pause while the list is filtered - Firefox should display about 8 preferences - for reference, this is what I have in Firefox 138:

I have the same config under the results of `ui.key`. The issue started at least a few months ago, it is not that new. Using your website:

ctrl+Z regardless of left/right:

• keydown => event.key: "z" + Ctrl
• keyup => event.key: "z" + Ctrl

shift+Z, regardless of left/right:

• keydown => event.key: "Z" + Shift
• keypress => event.key: "Z" + Shift
• keyup => event.key: "Z" + Shift

ctrl+shift+Z, regardless of left/right ctrl/shift:

• keydown => event.key: "Z" + Ctrl + Shift
• keyup => event.key: "Z" + Ctrl + Shift

alt+shift+Z, regardless of left/right shift/alt:

• keydown => event.key: "Z" + Shift
• keyup => event.key: "Z" + Shift

ctrl+alt+shift+Z, regardless of left/right modifiers

• keydown => event.key: "Z" + Ctrl + Shift
• keyup => event.key: "Z" + Ctrl + Shift

For alt, I had to change keys. Alt+Z was tied to AMD Software, so it only registered keyup. Others opened the menu bar. alt+C, both left and right alt. It doesn't even register alt???

• keydown => event.key: "c"
• keyup => event.key: "c"

For reference, in my other instance, it shows `event.key: "c" + Alt`

Possibly related detail: On the key test page provided, enabling "Log solo modifier key events (keydown repeats a lot...)" doesn't do anything on this instance for any modifier keys. On the other instance, Alt is the ONLY one that logs anything there. (Actually, Right Control and Right Alt do too?)

I searched for a new bug and couldn't find one. There was an older bug about international keyboards, but since that was from a few years ago, it wouldn't explain the recent change you're seeing.

You could file a new bug, and if you have extra time on your hands, you could run a Mozregression to triangulate on which change broke it.

• https://bugzilla.mozilla.org/home
• https://mozilla.github.io/mozregression/

You might also mention it on the Firefox subreddit (r/Firefox), as more Beta / DevEd / Nightly users will see your post there than here.

https://bugzilla.mozilla.org/show_bug.cgi?id=1965900

I forgot to ask whether you have exonerated your extensions and any startup scripts you might use. You can rule them all out at once using Troubleshoot Mode, but if it works as expected in Troubleshoot Mode, then you need to investigate the individual items.

Diagnose Firefox issues using Troubleshoot Mode - use Open, not Refresh, when given the choice

I forgot to ask whether you have exonerated your extensions and any startup scripts you might use. You can rule them all out at once using Troubleshoot Mode, but if it works as expected in Troubleshoot Mode, then you need to investigate the individual items. Diagnose Firefox issues using Troubleshoot Mode - use Open, not Refresh, when given the choice

haha, yeah I did that, as initially stated. I figured out the root cause (see the bug report), definitely seems like a bug.