I agree with all the other suggestions ... Rated 4 out of 5 stars
... but will add that I would like a preference setting in which I do not receive a warning. Seriously. When attempting to go over my limit, I simply would not be able to open another tab.
Many people - including myself - would get frustrated with such a feature at first, but I believe those who don't switch to a different preference would quickly get used to it.
A distinctive beep or flashing status bar icon would be a good idea though. Simply because of possible problems opening a tab when the user has yet to reach their maximum number of tabs.
e.g. - A user tries to open a link in a new tab. Nothing happens. He doesn't notice that he should be able to open one more tab, so he simply thinks he's reached his tab limit.
However, there actually was a problem with the link itself, or some bug with another add-on(s), or Firefox itself. Or maybe his mouse is close to death. So, yeah, a beep or flashing icon would be a very good idea.