The `.sh` file extension is now used for all Bash scripts. This ensures
the files are recognized as Bash scripts by the ShellCheck CI job, and
by editors/IDEs for improved syntax highlighting.
Two important notes:
1. The time it takes to download is negligble compared to e2e test runs
2. Since we cannot use environment variables in CircleCI cache keys we
can't cache the download correctly and have it update when we switch
firefox versions—this isn't the end of the world because of point 1
