Just the extension I was looking for. I have thousands of bookmarks, some dating back almost 20 years. I was never too keen on so-called "tags" to identify my bookmarks, and there was no such concept some years ago. Therefore, I heavily rely on the presence of a favicon to know where a bookmark is coming from and to classify them (a neverending task), as well as to identify broken websites.

In the past, I had to restore Firefox profile from backups because of various crashes, breaking favicons in the process. While keeping bookmarks was my main concern, it did detract from my experience with Firefox, not knowing the original website. Some websites also don't use clear webpage titles, further obfuscating their origin.

This application is very simple: I open it in the side panel, then click "Play": it will proceed on re-loading each bookmark five by five, closing them after timeout, catching favicons in the process and detecting broken bookmarks. Sure, it will use lots of CPU for a while, but probably won't saturate the RAM or crash Firefox. You can change the rather conservative default settings if you have a really powerful computer so as to speed up the process. One caveat: DO NOT close the side panel or switch to another view there: this will stop Checkmarks and you'll have to start all over again. My advice: let it run overnight.