A lot of features were removed like the control panel, variable width fonts and the tree bookmarks editor. It was 14 years ago, so it is hard to remember. The features removed weren't documented and reasons were never given. It just happened and it was very difficult to roll back because of the changes also required rolling forward or back the GTK+ libraries in debian.
There was already a browser called Epiphany and Galeon was changed to mirror the features in Epiphany even if that meant there were less features than in Galeon 1.2.x and removal of features was unpopular with the users.
https://noone.org/blog/English/Computer/Web/Browsers/Galeon%...
EDIT: I also forgot to mention that Phoenix, later renamed Firefox, was developed for Windows first. Linux development was considered an afterthought. It was recommended to use Mozilla or Seamonkey.