Goodbye Opera, Welcome back my Beloved Firefox Web Browser

I'd used Opera from May of 2008 to March of 2010, against my strong philosophy of open-source software use.

The 9.x versions come with lots of improvements of GNU/Linux version, and then with a greater plug-ins support.
At the time, Opera had got the greater efficiencies, greater standard respect and better result in some tests like ACID.
After months of use, I'd faced some issue (usually little ones but boring), for each one I'd reported a richly detailed bug.

After (almost) 2 years of use, I can observe that:
 - the GNU/Linux version of Opera still needs some improvements to better fit the OS components,
 - the greatest and the more boring is - sometimes - an important resources use for no obvious reason, dropping down the performance of the whole computer,
 - there is absolutely no visibility of reported bugs,
 - I've never received any return of my - at least 10 - reported bugs (never corrected, never referenced into Release Notes, never contacted for further information request ...)
 - they are late of 2 versions for GNU/Linux (although the "10.50 available shortly" mention is specified for month(s))
 - since creation of Unite, obviously - according to resources - there is less effort done on the "browser part" evolution
 - there is still some incompatibilities with "important"/useful WebSites like various Google services (gmail, blogger, documents ...), Bloglines ...

Because of the impressive richness of the Web Browser, it can't be replaced by everything.

During last 2 years, Firefox too has greatly evolved and lots of plug-ins too.
I'll post various information to switch from Opera to Firefox with some compromises (instead of loss).