That would be a bug. I use it all day with hundreds of tabs (sometimes thousands, but I'm getting better about pruning back) and don't see this. I know many, many people who could say the same.

If you have time, please file it: https://bugzilla.mozilla.org/enter_bug.cgi . If you're willing, include a profile ( https://profiler.firefox.com is pretty easy to use). You can look at about:performance (or in newer versions, about:processes, but I don't think that's in release yet.) When you file the bug, you will probably be asked if it still happens in safe mode (which would probably blame an addon).