How to determine bottlenecks in Firefox?

You will have to do it manually. First try running firefox in safe mode to make sure it is the add-ons slowing down your browser. If the browser still takes the same amount of time to start, you might want to try clearing your private data. Otherwise, you'll have to load the add-ons separately.

I once had a laptop that would do that. However it was a hardware issue - the keyboard would send keystroke to Firefox that I never generated. In the end I had to replace the keyboard.