I'm using Firefox 63.0.1 on Fedora 29 x64 with an X session (Wayland isn't possible for me due to display driver problems). This has been a problem since at least 1Password X 1.12.0 and also happened on Fedora 28. Unfortunately I didn't pay as close attention to Firefox versions so I don't know if it also happened on Firefox 62. When the extension popup is showing, the labels for categories (Login, Credit Card, Passport, etc.) are blank. The icon is showing, but the label text is not. Everything else seems fine, the labels for Watchtower, Password Generator, and Favourites appear normally and the category labels work correctly when they're clicked. It's just the text for just the category labels is missing for some reason.
1Password Version: 1Password X
Extension Version: 1Password X 1.12.3
OS Version: Fedora 29 x64
Sync Type: 1Password X