For a laugh, check out this bug[1] for the history page, extant since Firefox 11!, that means deletion is incredibly slow and tiresome. What makes me laugh even more is that history is stored in a SQLite database, meaning that deletion should be very easy and quick but they've decided to create some horrible extraction over the top that inevitably uses Javascript of some sort.
Just delete the stuff and refresh the page. Or don't, inform the user to close and reopen it themself, it would still be an improvement by a long way. Jesus wept, I know about Chesterton's Fence but how hard can this really be?
As this comment by a team committer shows, the direction of the product is wrongheaded and tells me all I need to know about why FF has fallen so far behind Chrome in market share:
> We understand this is still an issue, and we recognise that it is a pain point for people who do hit it. However, it likely involves a significant amount of non-trivial work to fix. At the moment our focus is on improving other areas of Firefox, and we don't have the time available to dedicate to this.