This is one feature that I did not imagine would ever work again in Firefox (after the death of Bookmark Favicon Changer, which does not even work in Waterfox). I specifically wanted to set bookmark icons, which is possible with this add-on so long as you first erase the affected URL from history (for anything that is in a bookmark it stands to reason that you would not object to any effects this may have on Firefox’s ranking for that URL in type-to-find). The ability to replace the icon for a whole site is interesting too, as it combats sites with no icon or a confusing icon (something I also deal with).

The only thing that I would note is that using only regular expressions does complicate it: I would default to substring prefix and allow regular expressions as an optional extra (preferably per-entry) to avoid the need to apply escapes unexpectedly. (As it happens I do know regex, but most folk using this surely won’t have a clue what they are.)

Thanks!