Note: the request for each link is done asynchronously, i.e. Firefox UI is not locked and frozen until the request completes.
v2.0: Script rewrite (based on johnp_'s contribution in the userscript 'Firefox for desktop - list fixed bugs in Mercurial'): now the REST API is used (one(1) network connection is made for all unique examined bug IDs). During this procedure, you may open the Web Console (Ctrl+Shift+K) to monitor progress. (screenshot)
v1.1 Now, a spinning icon appears at the end of each bug link during the title request procedure: During the request : After appending title: