How can I change the system languange on Firefox? Or how can I change the spell-check languange at least?

I've installed the Romanian version of Firefox and as I write and talk most of the time in english, I would preffer the use of an english version and with spell-check for english. As Romanian is my mother language, it would rather be less useful to spell check my own languange. I would apreciate any help. Thank you in advance!