Notepad (QuickFox)

Z 5 z 5 gwězdkow pógódnośony

wót Andrey Shemetov dnja Sept. 24, 2011 · trajny wótkaz

This is an excellent addon. Now I have shared clipboard between all computers which I am using. Used it for a week, didn't find even single issue.
Note that author recommends XMarks, but in my case it is working with standard FF Sync without any issue (I gave up on XMarks once they drop a password sync support).
Last, but not least, developer is very responsible and takes care about an addon.
Solid 5 stars.