If you are using firefox 3.5, and suddenly the right click context menu stopped working in firefox, then you can fix it easily.
Usually the problem arises because of the Yahoo Toolbar. If you installed some software that installed yahoo toolbar, then this can be a reason for disabled context menu. Or, may be, you installed the yahoo toolbar manually. In both cases, disable the yahoo toolbar to get the context menu back.
Another reason for this issue may be the Adblock+ addon. You may have to disable that as well.
How To Disable yahoo Toolbar in Firefox?
Go to Tools > Addons, find the yahoo toolbar, and click on uninstall. Once the toolbar is uninstalled, restart the firefox.
How To Disable Adblock+ in Firefox?
Just like the yahoo toolbar, uninstall it from the Tools > Addons menu.
I hope you got the right click context menu back. if you still have the problem, then use the comment form below to get in touch.