I don’t know, but I don’t think it is relevant to your initial issue.
Something you can try, but that will reset your addons, themes, and other configuration you may have made in Firefox, is to reset your profile (it should keep your bookmarks and passwords). To do so go to the menu button on top right corner, then go to Help → Troubleshoot Mode and it will restart Firefox, you could click Refresh Profile to reset/clean the profile, or you can also just simply click Open to start a clean Firefox momentarily.
See if Firefox behaves normally then.
On a side note when I start Firefox from the terminal, I do not have this locale error message, and I don’t have the GTK theme issues either. So even if I don’t think it is relevant to your issue, it might. So maybe try to properly configure the language/regional settings in your desktop environment (It’s all in System Settings for KDE in Manjaro → Locale), and maybe try another GTK theme (in Appearance → Application Style → Configure GTK theme).