I installed Bon Echo some weeks ago, and first thing I noticed was that some extensions weren't working anymore. Like, for example, webdeveloper toolbar, view source chart, etc... I could find a version of webdeveloper toolbar which worked in Bon Echo but I wanted to know how to do it. And here is it:
Basically each extension developer decides under which versions the extension will be able to work, and they define a range. For example: 0.7 - 1.5.* means an extension will work if you use a firefox version between 0.7 and 1.5 (quite obvious, no?). This means that any extension with this value won't work in Bon Echo, as Bon Echo's version is 2.
To fix this, just save the extension you want to install in any place of your computer. You'll get an xpi file which is actually a zip file. Let's assume it is called extension.xpi. Rename it to extension.zip and uncompress it... You'll get several files; the important one is install.rdf.
Open install.rdf with any text editor. You'll find an xml file with several configuration values, etc. Look for a pair of values called em:minVersion and em:maxVersion. Most of the extensions which do not work nowadays will have a em:maxVersion value like 1.5.. Just change 1.5. with 2, or 3, or whatever which is more than 1.5. Save the modified file.
Now we need to generate again an .xpi file for being able to install it in Firefox. So just compress back all the previous uncompressed files and rename the generated file.zip to file.xpi.
A way of installing this extension is just dragging file.xpi to firefox. The usual pop up warning you before installing an extension will appear; just follow the process as usual.