Once I was enthused about FS, BUT... Rated 2 out of 5 stars
Even though there is a homepage of FoxSaver, it is in no way related to the development of the AddOn. It completely looks like a commercial site... BTW, for a long time I realized some kind of memory-leak... then I realized, that the number of GDI-objects ('used' by Firefox) grew over the time to sick values. As a result I disabled my AddOns, one by one. When I disabled FoxSaver, the number of GDI objects remained growing moderate... (it still grew and I found more leaking AddOn's - though not all... sigh...)
Conclusion: As long as 'beautiful pictures' are more important to the developers than messages around bugs, there will be no change. Furthermore (and especially including these facts) FS looks like (as I suspected above) a kind of commercial AddOn and one should think about the sense behind it all. Ok, it's free, right. But it should at least be in the interest of the developer to find bugs and offer an easy way to post bug reports instead of seeking for images until the end of time..
As a consequece I have uninstalled it and won't reinstall it as long as there is no change. And if there is an update, I'll use sandboxie to install it using a sandboxed Firefox profile (and run this Firefox for several days constantly), so that any modification can be reverted easily. If it works clean then, I'll use it again in the regular profile - not before!