Add text variation to fx dl button. Bug 1079544.

A couple things:

1. `re.compile` is really only useful if you want to use a RegEx object multiple times. Since this is only once, I'd use the `re.search('regex', haystack)` form.
2. It looks like you're searching for `download-button-check-old-fx` surrounded by anything, so why not just use `download-button-check-old-fx` as the regex instead of capturing everything before and after? If you only want to capture that string surrounded by stuff other than more letters, use the word border escape sequence: `\bdownload-button-check-old-fx\b`.