Unbranded Firefox download must be hard-coded (for now)

[Hainish]

In this RUN command, you can see that for ESR and Developer Edition, the latest Firefox version is able to be programmatically downloaded from static links provided by Mozilla. Currently, however, there is no static link available for the unbranded version.

For this reason we have to manually parse the Extension Signing page to figure out what the latest version is, and hard-code that link into the Dockerfile. This is less than ideal. I've opened a ticket on bugzilla for Mozilla to provide a static link for the latest unbranded download, so this issue is blocking on that.

[dstillman]

Not that it's a good solution, but this is what we're doing as a workaround for our CI builds at Zotero:

https://github.com/zotero/zotero/blob/21d3ca298bd7d4e3d7f6f467c96b0c235cfcc824/.travis.yml#L19