(In reply to Germano Massullo from comment #20)
> I experienced the problem with both of them.

Did you check in Firefox about:support (look for "Driver Version") that it was
actually using your self-built Mesa?

BTW, I notice a lot of messages like

 [GFX1-]: ClientLayerManager::BeginTransaction with IPC channel down. GPU
process may have died.

in the Firefox output you attached. Have you contacted Firefox folks about
whether those are harmless, or how to get more information about them?