Traumhaft!
This is an excellent and useful extension that solves a common problem when cutting and pasting from PDFs and other on-screen text where the horror of extra line-breaks on pasting is very prevalent. It would also be great to have as a stand-alone tool.
Deserves to be more widely known and used.

Install, then make sure the remove line breaks icon (a small clipboard) is showing on firefox's toolbar.

Step by step use:
Copy text to clipboard.
Click the little clipboard item mentioned above. A green tick means that your text on the clipboeard is now ready to paste without formatting and surplus line breaks.
You can of course paste into other programs than firefox as well.