Existing Linux problem:

https://github.com/ibus/ibus/issues/2002
https://github.com/ibus/ibus/issues/2002#issuecomment-384883203
https://github.com/bitwarden/browser/issues/580
https://bugzilla.mozilla.org/show_bug.cgi?id=1405634

Solution:

download the ibus-gtk3 package from https://packages.ubuntu.com/artful/amd64/ibus-gtk3/download and install it. 

If you install it from terminal, in the folder where it’s been downloaded: sudo dpkg -i ibus-gtk3_1.5.14-2ubuntu1_amd64.deb

add a temporary exclusion for the package so it won’t be updated: sudo apt-mark hold ibus-gtk3
To remove the exclusion in the future, when the package will eventually be fixed: sudo apt-mark unhold ibus-gtk3

Restart Firefox.