I removed icecat 36.15 and installed FF 6.0.2.

But I how can i modify the bottom panel so that the Web Browser refers to FF.

Modifying openbox was easier

In many ways.
Making Firefox your default web browser.
You can use preferences in firefox or menu/settings/Preferred Applications/web browser or as soon Firefox ask you to be your default web browser.
If you want Firefox icon
Drag and drop from menu /Network/Firefox to the panel or right click
panel/Add luncher/firefox.

thanks franco,

managed to do it by adding removing launchers, although drag/drop and right clicking did not work.

Is there a file similar to openbox, where all settings are saved

There is settings manager and settings editor.
Maybe I did not explain drag and drop.
Click on the menu (dolphin-icon)
choose an app, in this case firefox
instead of click to run the app hold and drag and drop to the panel
a normal drag and drop.
right click on a panel /panel/add new items
you can just add luncher
you will see an icon whit a red x on the panel
right click on that icon/properties and configure the way you need.
Xfce has a lot of simple configurations.
You don't want icons on desktop like openbox
You can delete or
settings/desktop/Icons/Icon type and choose "Minimized application icons"
Once you do this and in settings/removable drives and media you uncheck
what you want not to be automatically mounted the memory needed drop
more o less to openbox.
In thunar/view select= show hidden files and you will see all config
cache etc.

Have fun.