I don't use Firefox any more, but what "buttons" are you talking about?
Round is not weird, so something else about their appearance is objectionable to you, I assume.
I never tried to customize Firefox when I was using it, and don't know if it's possible. Did you try to search for a solution?
Do you know why Safari is crashing? What did you try to prevent this? Perhaps it's fixable.
What version is your Safari? ...and/or: What is your OS version?