Firefox abandoning the more powerful XUL to web extensions resulted in a lot of extensions going away because there are not useable APIs. On the other hand, I would feel relatively safer to install a web extensions than a XUL nowadays because they are pretty well contained with permissions.
I miss XUL. Yes, you gave over complete control to the author of the plugin. But with a properly functioning rating and reporting system, bad extensions would be flagged and removed before you were statistically likely to use them. Also, I think that allowing users to open a "banking mode" with extensions disabled would be better than removing user choice entirely. But considering that browsers are quite literally a business, and you need funding to continue to develop one, all browsers had to remove user choice in order to not get hit by a "firefox is insecure" smear campaign.
DownThenAll! (or download managers in general) are still rather severely crippled, though – they can only download into the official OS "Downloads" folder, they can't skip files that already exist in the target directory...
