Patents last for 20 years in the US. To give you an idea of how rapidly the personal computing/internet boom has progressed, Mosaic is not 20 years old yet, Mosaic being the browser that marked the start of the www era.
If Netscape had been smart (and evil) enough to patent the hell out of their browser and then stick all those patents on a shelf for a while... they'd probably be more valuable than Apple is today.