Firefox, AdBlockPlus and Ctrl+Shift+V

I use Ctrl+Shift+V (Paste as Plain Text) extensively overall. However I experienced that in Firefox instead of doing the Paste it opens a sidebar called "Blockable items on current page".

It turned out that this is because of the AdBlockPlus plugin.

To restore the Paste as Plain Text functionality you'll have to

1. open the about:config page in Firefox, 
2. search for the preference name extensions.adblockplus.sidebar_key 
3. change the value of it to something else than Accel Shift V
4. restart Firefox 

After doing these steps, you should have the Paste as Plain Text functionality back.

The solution comes from this source.