Add firefox support.

[pylipp]

When creating the html, a title is prepended. Firefox uses it to set the tab text which is again what xdotool can search for.

Remember to set g:vim_markdown_preview_browser='firefox' in your vimrc.

[Luxed]

For me this change is not working because the name of the tab openned in Firefox is actually the name of the markdown file for some reason, so let's say the file is called test_file.md, the firefox page will be called test_file and my window manager's (i3) window will be called test_file - Mozilla Firefox. Please keep in mind that I use Firefox Quantum which may have changed the behavior somewhat.
I will try to see what I can do when I have time. Adding an option for firefox_quantum maybe be a good idea.