Try download from here(The personal web site of Chris Pederick and home of the Web Developer browser extension):

http://chrispederick.com/work/web-developer/

thank you

Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.

sorry, forget it, it redirect to firefox page... you allready check it.

Don't download the extension, but install it directly from the add-ons page.
That allows Firefox to do a compatibility check and install the extension.
That is needed because the install.rdf file has a maxVersion of 4.0.* and won't install without a compatibility update.

Thanks for the suggestion. I tried and the add on from the add ons page, but it still said it was incompatible with 8.0.01. Any other ideas?

Checking for updates doesn't seem to be working at this moment.
I did some testing and I do not see a response.

You can download the extension and edit the file install.rdf in the XPI file (ZIP archive) and change the maxVersion to 9.0 or more.

• http://kb.mozillazine.org/Editing_an_add-on_to_change_its_compatibility