Gread add-on! But, unfortunately not very easy to use at the moment, as I also have the problem with FF freezes upon using "mark all links visited" (FF 9.0.1, all other add-ons disabled).
Interestingly, this occurs on a fresh Windows 7 (Home Premium) installation - while there were no problems with the same browser on the old XP SP3 installation on the same computer. On the other hand, on another PC (this time also with XP SP3), freezes occurred as well; this time with FF 8 as much as with FF 9. It appears to be independent of the specific web page, although the number of links seems to have an impact on the duration of the freeze.
I have no idea about the cause but nevertheless hope it will be fixable (and fixed) as the add-on itself was great, extremely useful and is highly recommended - especially once the problems are resolved.