Rated 5 out of 5 stars

Easy way to make compatible with FF 38 and beyond (and don't waste time with "RequestPolicy Continued" - it sucks)

1.) Find "requestpolicy@requestpolicy.com.xpi" in your FF folder

2.) Rename the xpi to zip and extract it with your favorite unarchiving software

3.) Find the install.rdf file and open it in a text editor

4.) Change the following line:
<em:maxVersion>38</em:maxVersion>

To this:
<em:maxVersion>50</em:maxVersion>

5.) Save the file and zip the whole folder again

6.) Rename the zip into xpi and restart FF