A website can prevent or alter the use of the right-click menu. Some sites do it to prevent the use of copying. If it works on other sites maybe that's what they've done.

Some sites are built using Flash - which also stops the normal right-click options working.

Is there anything I can do to fix it? I know one of the sites it should work, because I sent them a question asking how to copy a picture (I own it) in my account because it's no longer on my computer. They are the ones who told me to right click the mouse to copy it. I imagine they wouldn't have suggested it if it wouldn't work.

Depends how the site is doing it. Preventing right-clicks can be done with javascript so turning javascript off (Tools > Options > Content) may stop it. But it may prevent the whole site working too.

There's no general answer - it depends on the individual site.

Have you tried googling? Others may have found a workround.

Do you have that problem when running in the Firefox SafeMode? http://support.mozilla.com/en-US/kb/Safe+Mode Don't select anything right now, just use "Continue in SafeMode."

If not, see this: http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...