|Which is correct?
Although Firefox has gained market share, ____ than 25 percent of Web users report that it`s their main browser.
Explanation: Because it`s a percentage of something quantifiable (Web users), fewer is the appropriate choice. We only use less to refer to something that is an undifferentiated mass — and humans tend to resent being referred to that way.
Martha Brockenbrough from The Society for the Promotion of Good Grammar offers more information in her Top 10 Grammar Tips. Also discusses that prohibition against ending with a preposition, using a Bonnie Raitt song as an example. (Would you want to listen to a song called Let`s give them something about which to talk?)