On the one hand, this seems like a real missed opportunity by Mozilla. As Chrome reigns in extensions that conflict with Google's business model, this is a reason to use Firefox. BUT - extensions are also often the cause of a slow and frustrating Firefox experience, which then leads folks to talk about how Chrome is better-performing/faster (I've been guilty of this myself in the past). Mozilla needs to make sure Fir…
I've not checked the source, but my guess is firefox stores the history in a SQLite db and when you select 500 history items then delete them, it's doing 500 DELETEs, maybe even without a proper index. It's seriously slow.
There seems to be a lot of low hanging fruit in Firefox, but not being employed by Mozilla I don't particularly feel like my thoughts or contributions are welcome. The last time reported a bug on bugzilla.mozilla.org (which is a pain in the neck) I was told that accessibility on MacOS wasn't important enough for Mozilla to care about (paraphrasing.)