This process will work for most addons, but I assert that it is in no way 100% reliable and in no way am I responsible for anything that occurs as a result of your usage.

Code
   1:# Change to your extension directory.
   2:# If you have multiple profiles, do
   3:# this for each one.
   4:cd ~/.mozilla/firefox/*.default/extensions
   5:
   6:# Download the Extension XPI install package
   7:# that you'd like to get working. 
   8:wget http://link.to/your/desired/extension.xpi
   9:
  10:# Use 7-zip to extract the install.rdf file
  11:7z x extension.xpi install.rdf
  12:
  13:# Use the editor of your choice (e.g emacs, vim, nano,
  14:# pico) to change the Firefox MaxVersion parameter
  15:nano install.rdf
  16:
  17:# Update the XPI package with our new
  18:# install.rdf file - voila!
  19:7z u extension.xpi install.rdf

This sequence of commands produces a "working" XPI package. Essentially, all that's changed is the MaxVersion field below:

install.rdf (original)
...
<targetApplication><!-- Firefox -->
	<RDF:Description>
		<id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</id>
		<minVersion>1.5</minVersion>
		<maxVersion>1.5.0.*</maxVersion>
	</RDF:Description>
</targetApplication>
...