You tried to use the common keyboard shortcuts in the Linux version of Firefox, with a keyboard layout other than us, and you realised they do not work. For example, Ctrl-C does not work when the Greek keyboard layout is active because Firefox receives Ctrl-Ψ (which is undefined).
This is a well-known problem affecting keyboard shortcuts in many languages.
How can someone solve the problem; Should Firefox for Linux be configured so that internally it would consider Ctrl-C and Ctrl-Ψ correspond to the same keyboard shortcut (perhaps in the language pack)? Well, the problem is that one would prefer a solution that is independent of the keyboard layout. You might be running a Greek localisation of Firefox with an active layout for Hindi.
The optimal solution is to have Firefox associate the keyboard shortcuts to physical keys (whatever that means) instead of the characters they are producing. Bug #69230/Mozilla has been there for quite some time although an acceptable solution is available in both GTK+ (GNOME) and OpenOffice.org. For example, in a GNOME application, both Ctrl-C and Ctrl-Ψ are equivalent.
So, what can we do now with the Linux versions of Firefox? Well, it is possible to write a Firefox extension that would intercept keys being pressed in a local layout and convert to the standard keyboard shortcuts Firefox likes.
Such a workaround is available for the Greek language, written by Athanasios Lefteris, at Mozilla και συντομεύσεις πληκτρολογίου σε Linux.
Currently the extension exists in the sandbox of the Mozilla add-ons, meaning that you are required to register (free) and also configure your profile to allow the view of sandboxed extensions (=in early stage of development, about to get accepted). It is desired to to try out the extension and write a short review. This will help to get the extension accepted as official add-on to the masses.
Many thanks to Athanasios!
There is an existing Russian version of the extension. It is expected that other languages will follow.