I just diagnosed and fixed a pausing issue with one of my Firefox extensions. The hard part is getting a reproducible case. But once you do, it's usually just a matter of doing a binary search through extensions, enabling and disabling half the candidates until the culprit is found.

A quick verification can be to start Firefox with the -profilemanager flag and create a new profile, and verify if it does or does not occur in a completely fresh profile.

In my case, pauses came from FoxyProxy regular expression pattern matching on urls where I was using:

  (.*\.)*

to match subdomains. The issue went away when I did:

  (?:.*\.)?

instead - non-capture, and of course noticing that . matched \. too.