A software license agreement is a contract between a producer and a user of computer software which grants the user a software license. Most often, a software license agreement indicates the terms under which an end-user may utilize the licensed software, in which case the agreement is called an end-user license agreement or EULA. EULA is very common under Windows and other oses like Mac OS X.
A free software license grants the right to modify and redistribute the licensed software for any purpose, both of which would ordinarily be forbidden by copyright law. So you get considerably more rights than most EULAs provide. Now Mozilla asked Ubuntu to display EULA first time you launch Firefox. From the bug page:
Mozilla Corp asked that this be added in order for us to continue to call the browser Firefox. Since Firefox is their trademark, which we intend to respect, we have the choice of working with Mozilla to meet their requirements, or switching to an unbranded browser.
I don't see a big deal here. If you don't like EULA in Ubuntu, try Iceweasel - rebranded version of the Mozilla Firefox program.