firefox add-on rendering issues! (they are driving me crazy)

the past couple of days i’ve been driven insane by a text rendering problem in firefox when using add-ons.
the problem appeared suddenly, so i wouldn’t be surprised if this is just user error on my part, but it renders my normal text as bold ONLY when viewing the site i am currently developing. you can see the comparison in the image above.
i’ve since just removed all add-ons, but that has left me feeling a bit vulnerable with no web developer, html validator, or firebug. i’m hoping that someone out there in the world will have some sort of suggestion for me, this thing is driving me nuts!