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

In [this](https://github.com/EFForg/https-everywhere-docker-base/blob/master/Dockerfile#L31) `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](https://wiki.mozilla.org/Add-ons/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](https://bugzilla.mozilla.org/show_bug.cgi?id=1294831) for Mozilla to provide a static link for the latest unbranded download, so this issue is blocking on that.