Mozilla is derived from the original Netscape sources and can be obtained free of charge from mozilla.org. But ever since Netscape was out it never supported true 64 bit architectures correctly. The OSF port of Netscape was done using the Digital C compiler which has a nice compatibility switch to turn off 64 bit addressing.
Linux uses gcc (or egcs) which does not have this switch so Mozilla has to be cleaned up reagarding bad pointer conversions. Most of the problems result from this.
A next problem is that Mozilla uses Motif as GUI toolkit. Since not everyone has a Motif license many users want to use lesstif, a free Motif clone (see http://www.lesstif.org/). Lesstif is really good by now but still has many bugs and causes Mozilla to crash or behave strangely.