(In reply to comment #17)
> > Thread 1 (Thread 0x7f2d032c2740 (LWP 3445)):
> > #0  0x00007f2d02ebfa8d in read () from /usr/lib/libpthread.so.0
> > #1  0x00007f2cfd97264a in mozilla::widget::GfxInfo::GetData() ()
> >    from /home/fred/Téléchargements/firefox/libxul.so
> 
> I wouldn't expect a blocking read on a pipe to use 100% CPU.
> Is this the same process that top is showing using the CPU?
> If it is, can you use 'H' in top to identify the thread, please?

Captured 2 minutes ago. When I launch Firefox :

7136 fred      20   0  309228  17808   8100 R 100,0  0,4   0:23.85 firefox

H in top :

7136 fred      20   0  309228  17808   8100 R 99,8  0,4   1:29.11 firefox 

It is what I get.