Feel free to read the msg from the gm dev forums below. To sum it up there are issues in later versions of firefox. The issue is pervasisve it is not just for our addon. So to make the story shorter I will summarize we will be supporting the use of firefox 19 and the gm located on our site. After extensive testing, we know these work so please use these as well.

Re: [greasemonkey-dev] Find Updates in script context menu not working?

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To: Greasemonkey Dev

On Sun, Jul 7, 2013 at 10:05 AM, xxxxx wrote:
> When I try to update a script, the “Find Updates” option is grayed out, see
> attached screenshot.
>
> With Firefox 22 I’m currently using Greasemonky 1.10beta1 after having the
> same issue with 1.9. I thought that I had read that this particular issue
> was resolved with 1.9.
>
> —
> You received this message because you are subscribed to the Google Groups
> “greasemonkey-dev” group.
>

Probably issue #1743, if a script has been modified, you can’t update it.

https://github.com/greasemonkey/greasemonkey/issues/1743

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