Purging Firefox removes global settings only, not the Firefox profile itself. To check whether it's an issue related to your Firefox profile settings, try creating a new, clean one:

mkdir /tmp/firefox-test
firefox -profile /tmp/firefox-test -no-remote &

/tmp/firefox-test can be removed afterwards. If the problem is persistent, you might want to take a look at your settings in the about:config URL, search for keywords like "mouse", "click" and "tab". Otherwise, you could check the prefs.js file in your Firefox profile folder which is located at ~/.mozilla/firefox/XXXXXXX.PROFILENAME/prefs.js. If you can't get any out of it, quit the Firefox instance using this profile folder and rename / remove this prefs.js file to reset your settings. If this still does not help, export data like your bookmarks and create a new profile using the Profile Manager which can be started with firefox -p.

Firefox

You can use some firefox options to both disable the default action which is to paste on clicking the middle-mouse button and also to turn on auto-scrolling.

First - in the Firefox - Preferences - Advance settings ensure you have the following options checked:

• Use Autoscrolling
• Use Smooth scrolling

Then in the Awesome bar type about:config

Search for *autoscroll and double click *general.AutoScroll to change from "false" to "true"

Now search for middlemouse

• middlemouse.contentLoadURL = false
• middlemouse.openNewWindow = false
• middlemouse.paste = false

Then restart firefox

When you middle click you should now see this symbol to scroll: