Configuration -> Common preferences -> Other -> Web browser
You can insert your browser there, for example:
 
/usr/local/mozilla/mozilla %s
 
The annoying thing is that you can't open a link when the browser is
already running (because then only the profile manager launchs). So I
use the following script which works with mozilla-clones:
 
 
#!/bin/sh
 
#browser="/home/georg/app/beonex-comm/beonex-comm"
browser="/home/georg/app/phoenix/phoenix"
 
if $browser -remote "ping()" &> /dev/null
then
        $browser -remote "openURL (${1}, new-tab)"
else
        $browser $1 &
fi
 
 
If the browser is already running then a new tab will be opened. Else
the browser will be started.
 
The script's filename is browser.sh - so I had to insert
/path/to/browser.sh %s
into the webbrowser field.