What OS?

Apologies. I thought when I put it in the input box where it says it detects my OS that it'd show in the post.

I'm on Win 10 Home 22H2

See if this helps. https://support.mozilla.org/hsb/questions/1367778

Thank you.

Tried the xulstore.json reset/rename

No change in behavior.

Only workaround that currently works is pressing the Windows key or taking Firefox out of maximize and re-maximizing. Without doing either of these things, Windows taskbar cannot be accessed. It's admittedly minor but is a bit annoying

Yeah, Windows 11 taskbar is very annoying not being able to change the size of it. I'm rarely in windows anymore.

I'd be more willing to place the blame on Windows here since it often is to blame but Firefox seems to be the only application that has this issue

I have the same issue in Windows 10. And the issue recently appeared in 133. I am currently on 136. I have found if I left click on the area around the bookmark bar or the menu bar the issue disappears - only for that session. Only app that does this. Also does not happen on Firefox derivatives such as TOR or Mullvad browser. Weird. Minor. PITA.

I'd be more willing to place the blame on Windows here since it often is to blame but Firefox seems to be the only application that has this issue

Quoting myself to fix this statement.

This apparently is not a fault with Windows.

What appears to be happening is that if Firefox launches full screen, it does exactly that. On a 2K monitor with native 2560x1440 resolution, Firefox will launch at 2560x1440. You might think, well of course, that's what full screen means.

However, for the autohide taskbar option to function, Windows uses 2 pixels as the buffer to detect the mouse cursor there intending to unhide the taskbar. As a result, a full screen window is actually 2560x1438.

After launching a maximized Firefox, if you shift focus away from it and go back (either alt tabbing or minimizing and restoring or even doing something like popping up the Firefox About window), Firefox will behave like every other app and the fullscreened Firefox will now be 2560x1438.

So it seems to be a Firefox issue that it launches without the 2 pixel buffer.

To note, this issue still persists in the current v136.0.2

Michael - Thank you. That is the clearest answer to the problem I've run into. Not that I'm any closer to a solution but I at least have a direction I can go with. I wonder why Mozilla can't resolve this issue. It ought to be fairly trivial since no other application seems to have this issue.