Firefox has tools to figure out which tabs are causing problems, try about:performance. Any freeze of minutes is generally caused by a memory leak in the javascript of a particular page.
I thought the recent work on multi-process stuff would have gotten rid of that? The browser chrome not being affected by in-page issues seems like a fairly important goal there...
I have about 200 tabs on my work machine and something even worse on my personal one and have never seen this, so unless you're a serious tab hoarder I doubt it's that.
