Another user mentioned this recently however it works for me on Ubuntu 23.10.

Another user mentioned this recently however it works for me on Ubuntu 23.10.

Unfortunately, this didn't help.

Try installing a different package format like flatpak.

I've got a similar problem for over year now. I'm on MX Linux, with their packaged version of Firefox using the official binaries (firefox/mx,now 125.0~mozillabinaries-1mx23+1) as well as fcitx5-mozc. (I've also tried the ESR version from debian and I think even the flatpak) IME works like a charm (almost) everywhere else, including Thunderbird and the Tor browser. In Firefox however, it's like fcitx wasn't installed.

I took another swing at this and discovered that on a fresh Firefox profile, Japanese input does work as expected! So there has to be some setting somewhere in my profile that breaks IME. Incidentally, composing accented letters using the ` key also doesn't work in my old profile (I can't type accents at all, seemingly nothing happens when I press that key), but functions fine in a new one.

I've searched in about:settings and about:config for "input", "key" and "ime", but nothing caught my eye as a possible culprit. Please let me know if you have any idea which preferences I could try resetting! This profile has been with me for 9 years and 3 operating systems, and I'd like to avoid migrating my settings, add-ons and userchrome to a new one.

TL;DR: If your fcitx/ibus input methods don't work in Firefox despite everything being set up correctly and working everywhere else, try the following: - Go to about:profiles - Create a New Profile - Click the "Launch profile in new browser" button on the new profile - See if your input method works in the new profile

Backup your profile folder and try renaming the prefs.js file to see if it is caused by an about:config setting.