Did you install from here:

https://addons.mozilla.org/en-US/firefox/addon/wot-safe-browsing-tool/

The Mozilla Add-ons team does some testing of add-ons hosted on that site for safety and non-crashiness, but since the add-ons depends on an external service, there's really no way to test it under all conditions and for all time.

I have not used WOT. From what I have read, there sometimes are "false positives" where members flag sites that aren't really dangerous but are offensive in some way. However, WOT or something like it probably is more helpful than not.

I will complete jscher2000 with my personal point of view.

I've been using WOT for a while and I can tell it's a good and safe add-on. It is a website checker that tests, with community support, if a website is safe for browsing. It has only one problem, it makes the navigation a bit slower. But if that's not a problem for you, it is a good safety and even if you want parental control filter.