It works.

Previously it was possible to assign a favicon to all of your bookmarks, but that favicon would only show up on a tab for exactly that URL. The moment you started browsing the bookmarked site, let's say example.com to example.com/whoops/, it reverted back to the original. At least that's how it was for me with 3 different add-ons.

Now, with this extension, most bookmarks don't accept custom favicons (which I know is due to Firefox's limitations), but they do show in tabs even when I browse the site. Subdomains need extra favicons.

It seems Firefox users sadly can't have custom favicons for both bookmarks and tabs. The way it is now, Favicon switcher does a great job.
One suggestion, though: Please enlarge the add-on window just a tiny bit. Right now, I have to hover over URLs to identify them. With 100+ custom favicons, that can get a bit tiresome.

Works great!

It works well for most things, but when viewing pdfs using pdfs.js, it goes back to the original favicon.

Sadly doesn't work on the bookmark toolbar. Also hasn't passed Mozilla's security program which implies to me this project is no longer maintained.

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.

Works well when I apply new images as instructed and is easy enough to figure out on your own if you don't read instructions (like me), but would like to see a feature to replace bookmark favicons in the future. If that's possible, it'll be an instant 5-star once it was available.

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

Doesn't accept any of my .ico files. They all worked with previous versions of Firefox. The error "not an image file" is not specific enough to know what it means.

-update- (Feb 17)

The extension now supports my .ico files. Thank you Saber.

Unfortunately the extension doesn't effect the Bookmarks Toolbar's default icons for html files I have on my harddrive which is what I was hoping for.