SecureProxy for Firefox on Ubuntu 18: can’t input credentials, extension disappears

Existing Linux problem:


download the ibus-gtk3 package from 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. 

Other tutorials: