You can try some of these add ons https://addons.mozilla.org/en-US/fire.../?q=different+language+tabs&appver=29.0&platform=all

However I wonder if there is a build in feature? https://support.mozilla.org/en-US/que.../949545

You can change the language settings with these references: http://pchelp.ricmedia.com/change-mozilla-firefox-language-sett...

Thank you for your reply!

I think I've accidentally marked the problem as solved, but in fact it's not.

What is needed is that the firefox remember different input methods (languages, for instance, En or Fr) for different tabs so that when I switch between several tabs I don't need to change the layout.

Chrome remembers my language settings for each tab, so it's not always one language for all the same tabs there. I haven't found such an option in Firefox so far.

Hope it's clear enough. Thanks in advance.