Thank you all for the response.
@qufighter
I want to enable other users to run the script optionally on some
pages which they desire to run it on. So I do not have control on the
page to create a button or any other element.
@Anthony
Bookmarklets can work for me, but I would keep this as a fallback
option.
@BD
A menu for greasemonkey to enable such selective running of scripts
would be great! Do you know if I can provide such a button by creating
a firefox extension? I can try to convert my script to an extension
using User Script Compiler --
http://arantius.com/misc/greasemonkey/script-compiler