Works great!

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!

I was using something similar until web extensions came along and deprecated it. I've been looking for a replacement ever since. It's great that you came up with FS. Extremely easy to use and gives more meaning to those entries that have no favicons or some generic icon. However, FS doesn't seem to work for the about: sites.

I've only got need for replacing the tab icon with a png-file, so I've had no problems.