For approximately two months (after an upgrade to version ~45) my Firefox dialogs have had extraneous brackets (see diagram). This happens on both Windows and Ubuntu. Is this a Firefox bug, a change in the Firefox API that I need to report to plugin authors, or something else?
I've reported such a problem with Firefox 53 HU. It's caused by invalid access key configuration. Mostly in localized files, I guess. It's a Mozilla bug, so you should report it in their Bugzilla if it comes up again.