With the caveats that it doesn't apply to re-tweets and that, having kicked it off, you pretty much need to leave the Firefox window in the foreground with the focus for it to run reliably, I found that this works very well and successfully deleted >15 years of old tweets.

It still works (8/2025) but is very slow going one by one - so only a good solution if you're more patient than me or have much fewer tweets :)