Try disabling Adblock, then give it a shot. Although I use Adblock and Firefox as well, I myself was able to copy and paste to the clipboard with one caveat - when I initially opened paint, I could not rt.click and paste, I had to use the "Paste" button from the Ribbon bar...which pasted the image. Weirdly, after I pasted it the first time with the button, I was then able to use the rt.click menu to paste.
If that doesn't work, here is the way I would go about troubleshooting this problem.
1. Make sure you have the latest versions of your browsers, re-install the latest if you already have it.
2. Start Firefox in Safe Mode - click the menu button (the three horizontal lines button - top right of browser), then the little help button at the bottom, choose "Restart Firefox with all add-ons disabled". If it works in safe mode, the problem could be an add-on, which you will need to disable one by one to find the culprit through a process of elimination.
3. Do you have any kind of AV software running? Try disabling it.
4. Try using "Save Image As..." in the context menu (rt. click menu) - does it work that way?