Edit menu items always enabled on macOS (since FF 153)

I noticed this because "Undo" and "Redo" are now always enabled, even when the undo stack is empty.

I looked into it and found that upstream changed the behavior so that the Edit menu items (Undo/Redo/Copy/Paste, etc.) are always enabled on macOS (https://bugzilla.mozilla.org/show_bug.cgi?id=2036608).

The reason for this upstream change is given in the Bugzilla issue linked above: without it, Cmd+X/C/V/Z/A stop working inside native macOS sheets (which applies to Zotero as well, e.g., the "Save As" dialog shown when exporting). I implemented a quick workaround and confirmed that this is indeed the case, and it makes those dialogs unpleasant to use.

I'd leave this as is, unfixed, and accept the trade-off. I'm mainly raising it here for awareness, and hopefully upstream will fix it eventually.