When I want to do multipe searches at once I use a neat gem from firefox bookmark feature: keywords.
I assigned "g" to google's bookmark, so when I type "g whatever" in the address bar, firefox will open the google search results for whatever.
Then I have vimperator
installed which lets me open several urls/addresses at once, by seperating them with comma followed by a slash (may the normal address bar from firefox lets you do that too).
So now I can do "g whatever, g some more, g and this too" and it would open three tabs with different search results. Taking this even further and assuming google was my default search engin (it is actually duckduckgo) entering "whatever, some more, and this too" would have the same effect.
If the normal address bar can do that, it might be worth a try. vimperator
make some changes to the user interface that aren't appreciated by everyone but loved by others.
So far I have come very far with the basic bookmarking feature of firefox