After installing Firefox 1.5 on a brand new laptop, I tried reinstalling trusty old ChromEdit. Unfortunately it doesn't install on Firefox versions later than 1.0.7. After some digging around, I found this useful FAQ:
The ChromEdit
extension is recommended, but I can't find a version that will install
on newer versions of Firefox or Thunderbird.
The last time I checked, this extension was not updated to install on
Firefox 1.5 or Thunderbird 1.5. You can download a repacked,
bumped-version copy of this extension from here.
Right-click on the download link and select "Save Link As..."
For installation in Firefox, open the file from the File menu. For
installation in Thunderbird, install the file from the Extensions
Manager. This repacked version is available for your convenience.
Absolutely no support will be offered, including installation, removal,
or how to use it.
ChromEdit 0.1.1.1 Repacked
(source: http://www.tom-cat.com/mozilla/faq.html#chromedit)