I use Firefox web browser, currently on Ubuntu 10.04 LTS. For many years I have set the config option middlemouse.contentLoadURL to true so that middle clicking anywhere in the page (that does not accept input) will load the URL that is in my clipboard.
After restarting my web browser somewhere near the end of January 2012 I found my Firefox 3.x had been upgraded to Firefox 9.x. Also the middle click behaviour no longer worked.
Perusing about:config showed that the option had been set to false again. I set it back to true but on restart of the browser it was set back to false. A bit of searching about found various suggestions about forcing it in my user.js file, but none of those worked either.
Finally, in desperation, I did a search of every file beneath /usr for the string “middlemouse”. Lo and behold:
… pref("middlemouse.contentLoadURL", false); //setting to false disables pasting urls on to the page …
Commenting this line out once more allowed me to change the setting myself.
It seems this this override was discussed by Ubuntu as far back as 2004, but it only became something that I could not override upon the upgrade to Firefox 9.
I reported a bug about this, and one of the comments seems to suggest that the method Ubuntu uses to change these settings has changed because they were breaking Firefox Sync, and that this outcome (overriding middlemouse.contentLoadURL) is not as bad as breaking Firefox Sync.
Even so, I would suggest that this outcome is very confusing for people and that as middlemouse.contentLoadURL is a popular setting which is easy to change, it should not be overridden in some obscure file.
As of the recent upgrade to Firefox 11, the file with the override in it has now moved to /usr/share/xul-ext/ubufox/defaults/preferences/ubuntu-mods.js.