Would give a negative-number rating if I could Rated 1 out of 5 stars
(1) Does not run on your computer. File conversions take place on their (Nitro's) server. Definitely a no-no for confidential files.
(2) Cannot handle files on your local filesystem; must be on a web-site (so their server can access it).
(3) I tried my standard test of a PDF converter -- I have an admittedly slightly tricky file: Something I made years ago in FrameMaker, using just plain text in a single standard font, some with strike-through effect and some with a double-strikethrough simulated by combining strike-through with a line, and also some lettering at an arbitrary angle. FM output was in PostScript, of course. GhostScript was able to convert it to PDF, rendered pixel-perfect when viewed in Adobe Reader (I haven't tried other readers).
Tried "Convert PDF to HTML" under this beastie. Result looked like complete cr*p: lines in *worng* places, worng font, text not angled that should have been, etcetera...
Granted, other free PDF-converters I've tried have not fared much better in this test....
(4) Most annoying of all is the constant push of adware urging you to try/buy their "Professional" version. (If this version works so badly, I'm not even going to bother with their 2-week -- too weak-- demo).
I don't know if this last mis-feature violates the letter of anything, but it definitely goes against the spirit of FireFox Add-ons as I understand it.
Bottom line: I'm sorry I wasted my time with this. Don't waste yours.