We are currently tracking a bug with Ubuntu 18.04's ibus package which causes password fields to not focus correctly in Firefox. The 1Password X popup (which is opened by hotkey or by clicking the 1Password logo in your toolbar) can't remain open without maintaining proper focus. This is preventing 1Password X from being able to be unlocked from the popup in Firefox. So far we've only seen and reproduced this issue using Ubuntu 18.04.
This bug was reported to Ubuntu by one of the Firefox contributors and in our testing affects all versions of Firefox on Ubuntu 18.04.
There are a few ways you can bypass this issue until Ubuntu 18.04's ibus package gets appropriately patched:
Please let us know if you see this issue on any other distribution or version of Ubuntu. We are proud to support Linux and rely on our amazing Linux users to help weed out OS specific issues.
We will keep this thread updated with any pertinent information that may arise.
UPDATE (1/10/19): Ubuntu has reported that this issue has been resolved.