Replace the URL bar with a tool to support more than just one command? Isn't that what keyword searches are for? I find the firefox URL bar to be extremely useful when combined with keyword searches. Here's how:
Go to any search field for instance the google search box, right click and choose "Add a Keyword for this Search...".
Give the search a single character "keyword" (eg g for google).
Now when you want to do a search you can do the following sequence:
Ctl-L # access the URL bar
Keyword [SEARCH TERMS] # eg "g slashdot" will perform a google search for slashdot
These are some of the keyword searches I use most often:
p for pubmed
g for google
gs for google scholar
gm for google maps
w for wikipedia
d for duckduckgo
ed for english dictionary
sd for spanish dictionary
The URL bar is by far the most useful feature of Firefox!