Try this

• http://kb.mozillazine.org/Firefox.exe_always_open
• https://support.mozilla.org/en-US/kb/firefox-already-running-not-responding

From those links you gave me, they were based in the fact that Firefox.exe is always open, but that's not the case, because there is no firefox application in the windows taks manager :S

Are you using Themes?

try in safemode

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

thanks, problem solved! :)

any advice on how to get cpu usage lower?

I am trying both mozilla and chrome, and they both get a lot of time 100% CPU usage, taking lots of memory.. (and mozilla is the one who takes the most, comparing both)

Try with limited number of tabs.

• Check for Add-on Updates

Try this

• http://kb.mozillazine.org/Firefox_CPU_usage
• https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix