How to make older Firefox extensions compatible with newer browser versions?

Have you ever had a Firefox extension that is no longer working with the most recent version of the Firefox browser that you have installed? Well, your agonies are gonna end soon (probably!), because this is a guide on "how to use your older incompatible extensions in newer versions of the Firefox browser".

Caution:
I'm gonna warn you, that any change you make to those extensions is your responsibility and by applying these methods, you also accept the risks involved. I suggest that you first make a backup of your Firefox bookmarks and then use the guide. By the way, don't be discouraged by my warning, as we all know that we're not fiddling with a bomb or something!

So, here we go.

I - First of all, you need to have the file of the extension add-on, which is a file of type .xpi. If you've just installed the extension and don't have it's file, look for it in Firefox Add-ons site and if there, right click on it's link and save it, and if not there, Google it's name and cross your fingers. But if you have such a file, continue reading.

II - Change the extension's .xpi file type to .zip. This way you can access it's contents. Now unzip the file somewhere and go to the containing folder. There, you can find some files, one of which is named install.rdf. Open it with the WordPad text editor.

III - Here it gets a little tricky. Find the following tag:
<!-- Firefox -->
or if you couldn't, look for this:
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>

The first one is actually just a comment, so if you didn't find it, don't worry. What you've found, may look like this (Notice that the mentioned tags are in bold) :

<!-- Firefox -->
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>1.0</em:minVersion>
<em:maxVersion>3.0</em:maxVersion>
</Description>
</em:targetApplication>

Sometimes there are some other tags too, like <!-- Mozilla Suite -->, but ignore them. As you can see, there's a tag <em:maxVersion>, which is the culprit that forbids you from installing your beloved extension. Yes!! Just a simple tag! So what to do? Huh! Easy. In my example, there is a 2.0 in the maxVersion tag. It effectively limits you to install the extension on up to version 2.0 of Firefox and not more. Just change it to 3.0 or 4.0 and you know better than me that what this change means.
Here's the modified portion of install.rdf, with the maxVersion changed accordingly (shown in bold):

<!-- Firefox -->
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>1.0</em:minVersion>
<em:maxVersion>3.0</em:maxVersion>
</Description>
</em:targetApplication>

IV - Save your modified install.rdf file. Compress the files again in a .zip file (Remember to compress the files and not the containing folder). Revert the .zip file type back to .xpi.

V - Open the Firefox browser. Go to Tools -> Add-ons. Now just drag the modified extension file and drop it into the Add-ons window (If other windows are in the way, minimize them, even Firefox itself). Restart your Firefox. Whoops!