However, I have no time and no «lust» to try this upgrade and hope, to find a solution how I can kill this request repeating itself every time I touch Synaptic.
Does that mean that Firefox is shown as an installed software in Synaptic. If it is, you'll keep getting "update advice". There's a way to lock a version in Synaptic described over at askubuntu.com: _http://askubuntu.com/questions/9607/what-does-lock-version-do-. But as they point out, it's strictly Synaptic. Other update methods may still show you the update. The suggestion by bodhi.zazen here may get around even that:
(Apologies in advance for linking to a "ubuntu" related site.)