UPDATE: The official Live HTTP Headers add-on now works with Firefox 4:
Download Here: http://livehttpheaders.mozdev.org/
Note this is a temporary solution, and worked for me with Firefox 4 b12:
- Download the .xpi file (Use right-click save as or Firefox will try to install the add-on)
- Rename .xpi to .zip
- Unzip the contents
- Edit install.rdf
- Under the <–Firefox –> Section change <em:maxVersion>3.6.*</em:maxVersion> to <em:maxVersion>4.0.*</em:maxVersion>
- Rezip the files
- Rename it back to .xpi from .zip
- In Firefox do file open, and open the .xpi file.
The add-on should install, just restart Firefox and use as normal.
Note: It works on my machine, however your mileage my vary. Obviously, I recommend that you keep an eye out for an official update and install the proper version as soon as it’s available…