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The Mozilla appsuite was released as a 1.0 product in 2002 Firefox was released as a 1.0 product in 2004

The last GIF patents expired in 2006 (the first, in 2003 after the 1.0 version of the Mozilla App Suite)

Looking further, there were no patents on GIF decoders (from < http://www.gnu.org/philosophy/gif.html >). So Mozilla was completely in the clear.

There was only one patent, for LZW, but it was held in multiple countries. It first expired in 2003, and had expired in the last country by 2004.