Add make target for running the extension in Firefox
Add make target for running the extension in Firefox

This change makes it easy to run the extension in Firefox without
packaging it into an extension. Use the `make run-firefox` command to
build the extension and open it in Firefox.

mfburnett/feminist-news-feed-eradicator,jordwest/news-feed-eradicator,mfburnett/feminist-news-feed-eradicator,anfedorov/news-feed-eradicator,mfburnett/news-feed-eradicator,anfedorov/news-feed-eradicator,mfburnett/news-feed-eradicator,jordwest/news-feed-eradicator,jordwest/news-feed-eradicator