<pre>
fossil ui gnupure.fossil-scm
sh: firefox: not found
(hangs)
</pre>

It would be good if fossil had a built-in list of web browsers to search for in the user's $PATH. Debian, Ubuntu, gNewSense, and probably other .deb-based GNU/Linux distributions have the concept of the alternatives system - "x-www-browser" exists, which the user can set to be whatever they want, but usually has a sane default. So IMHO a good order to try would be:

<pre>x-www-browser icecat iceweasel firefox konqueror dillo www-browser elinks links2 links-hacked links w3m lynx</pre>

Reasoning: If x-www-browser exists, it should be what the user wants to use. If icecat exists, it is most likely because the user has made a choice to use it, as it's not default in any distros I know of. If iceweasel exists, firefox probably doesn't, as it is usually just in Debian instead of firefox. Next, firefox, then if that doesn't exist, try KDE's browser, konqueror, and then dillo, a lightweight browser which is the only option on some USB/CD bootable distributions. If none of those exist, the user may be on a box without X-Windows; try www-browser, which should be what the user wants to use, followed by the web browsers that can do full-screen GUI to frame buffer, elinks, links2, links-hacked, and links, followed by text-only web browsers, w3m and lynx.