As is written here
the add-on CSS Viewer works with Firefox 1.5 - 3.0.* and is not available for Firefox 4.0.1
However there is a way to install it on Firefox 4 as following:
1. Download addon-2104-latest.xpi file to your PC by right click on it and selecting save link as
2. Open it with 7zip
(without to decompress it) and edit the included file install.rdf with a txt editor
3. On line 18 change this <em:maxVersion>3.0.*</em:maxVersion> to this <em:maxVersion>4.0.*</em:maxVersion> and save the install.rdf file
4. Then choose to open the (updated in this way) addon-2104-latest.xpi file with Firefox and you will see the message of installation on which you will continue
5. After installation restart Firefox as it is suggested and clicking on menu View -> Toolbars -> Customize you will find CSS Viewer into there
The above instruction is not a mine idea and is written here
at Reviews, but on replacement of install.rdf file, at line 18, this which suggests to change 3.0.+ to 4.0.+ did not work. So i tried the replacement 3.0.* to 4.0.* and worked.
So i place it here for those one they may want to try it.