it works now.. don't know why it stops working sometimes...
I also found this to be useful, as I often use Remote Desktop:
The following is a troubleshooting guide for when your clipboard stops working while using Mozilla Firefox.
NOTE: If you recently installed Mozilla Firefox, and
your clipboard has never worked, your computer is probably being
affected by malware or spyware of some sort. Try running these
recommended programs for removal: Ad-Aware and SpyBot.
The clipboard can, literally, become unfocused if you open multiple
windows using a desktop or the quick launch shortcut, and this can
break clipboard tasks in the text fields. Try minimizing the windows
and then maximizing them again to reset the focus if you encounter this
issue. To avoid this issue completely, launch new windows using the New Window from the File option on the taskbar. You can also use the shortcut CTRL+N.
If you are using a rich text editor and the clipboard functions
(copy and paste) are not working, it is probably because the editor on
the Web site you are using is only made for Internet Explorer. Mozilla
Firefox disable these specific types of editors from the clipboard
function for security reasons, and many Web sites are starting to
provide more secure editors because of security issues.
The clipboard functions can also stop working if you are using
Microsoft Remote Desktop. Shutting down the remote desktop usually
restores the functionality.
http://grok.lsu.edu/Article.aspx?articleId=4674