Mozilla Firefox uses proprietary plugins to browse all multimedia contents. Free software plugins exist, but they ignore them. The only way to use free software plugins + Gecko engine is the IceCat web browser:

http://www.gnu.org/software/gnuzilla/

IceCat is a fork of Firefox. It has some privacy enhancements, but the most important difference is that IC doesn't use proprietary plugins. IceCat is the official GNU web browser.

Free software plugins are:

PLUGGER: supports PDF, all multimedia formats (including Ogg Theora and Ogg Vorbis) and many other formats (OpenOffice documents, Abiword documents, etc... and even some strange formats like Commodore64 audio files!). Konqueror doesn't need it, because it has the K-Part KDE component.

http://fredrik.hubbe.net/plugger.html

GNASH: GNU software to play Flash files. Doesn't support very recents versions of the swf formats, but it's still under heavy development. Also supports Konqueror (requires konqueror-plugin-gnash and klash).

http://fredrik.hubbe.net/plugger.html

SWFDEC: an alternative software to play Flash videos. Some people say it works better than Gnash, but couldn't let it work with Konqueror.

http://swfdec.freedesktop.org/wiki/

HOW TO USE ICECAT AND FREE SOFTWARE PLUGINS:
http://wiki.gnewsense.org/Documentation/IceCat