benj1 wrote:have you tried the --new-tab flag?
don't have firefox or iceweasel at the mo but im pretty sure theres the command in the man page.
you may need a small shell script to the effect:
if pgrep iceweazel > /dev/null; then #check if iceweasel is running
iceweasel --new-tab $1 #if so open in new tab of existing window
else
iceweasel #else start new process
sleep 2 #may need a delay, don't know, fiddle
iceweasel --new-tab $1
fi
pretty psuedo codish, don't know what iceweasel processes are named (iceweasel-bin ?) or the exact name of the flag or whether that sleep is needed, you get the idea tho.
You know what we need! A "banging head against the wall smilie" - man page - why didn't I think of that --- I check the man pages for 'everything' --- obviously NOT! 
With "iceweasel.txt" I now have 101 man pages in ~/Documents/Manuals
-new-tab URL
Open URL in a new tab in an already running Iceweasel process.
But that is "an already running Iceweasel process" but at least I'm getting closer.
Interesting, in my local homepage I have a link:
<a href="http://crunchbanglinux.org/forums/" target="tab3">
I tried 'target="new-tab"' and it didn't work. I have up to - target="tab12" in the page.
target="new-tab" works, it opens a second site in a new tab. If I select yet another link, it re-uses the second tab - that's why the tab1, tab2, tab3 ... tab12 
That script looks interesting but how would I run it by pointing a mouse to a link in an email?