Assegnate 4 su 5 stelle
1) There is no constant effect: font is replaced until page be reloaded.
2) There is no global effect. You replace font on one page of the site, and the other page of the same site has fonts you just tried to eliminate.
3) There is no universal effect at all. Once you try to wipe out Comic Sans from your life, you must replace it on each and every site you meet, each and every time you visit any page.
Conclusion: useful for designers to check layout or to analyse competitors. Not usable for convenient Firefox users.
Sorry for the confusion, End User, but Font Finder isn't meant to be a permanent on-page replacement tool. Its meant for debugging and tuning (much like the Developer Console & Firebug.)
If you're looking for a more permanent solution, maybe something powered by GreaseMonkey could serve your needs.