In Ubuntu you can extend your clipboard functionality with an application like Parcellite Clipboard Manager, but if most of your work is concentrated in the Firefox browser why not get a Firefox specific clipboard – Clipple. By default Clipple extends your clipboard with 15 entries – but that’s just for starts, you can increase the number of entries but not without making your memory grumble, you can as well limit each text entry to a maximum number of characters.
Clipple is capable of saving your clipped text between Firefox sessions, if you want it can also copy your “passwords dots” in dialogue boxes and paste them as cleat text. Though Clipple is not a system wide clipboard manager, it stores in its memory all the text you copy out of Firefox (in text files on your system or other browsers), ready to paste them in Firefox when you do the mouse right-click -> Clipple Paste.
Clipple is an experimental Firefox extension – Its a free download for all platforms (Mac, Linux and Windows)