The file applies patches on sites or scripts (most likely JS libraries) that aren't compatible with Opera and it is automatically updated.
I don't agree with this method, because I do think Opera support should be requested to site developers and it should not be Opera to cope. I also don't agree with it because it changes Opera's default and because the two guys who apply those patches have too much power in their hands.
So I have disabled the "browser.js" file to see what happens.
Note that value "1" forces "browser.js" update and value "2" is the default setting.
There are chances I won't notice any difference with or without the "browser.js" file.
That is the way things should be.
If so, I plan to keep it disabled.