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1.5 was known for its abbility to be utilized for emulation and homebrew.
Sony wasn't pleased when they heard about it so they released 1.51
None of the version 1.# really did anything other than add languages and make security fixes. 2.0 came and introduced a feature to put your own background and surf the web with a built in browser. The other version 2.# added things like RSS support amd Location Free support, and eventually games began to require version 2.0. Now using a copy of Grand Theft Auto Liberty City Stories you can run software to downgrade and gain the functionalities of 1.5
However there are also loaders that allow you to run mostly any game in version 1.5
If you get a PSP with 2.5 firmware, then there is currently no way to go back. However, there is a "back door" which can be exploited in GTA which allows homebrew software to be run. It's already covered in another thread.
No, in fact the more you worry about firmware the less games you will be able to play, as the latest ones all have mandatory firmware upgrades. The device does what it's intended to do, don't worry about firmware.