Welcome to the Forum, arbaca!
If you choose Firefox and Thunderbird, making Firefox your default browser, and Thunderbird your default Mail client, then Firefox will open if clicking on a link in Thunderbird. SeaMonkey will do the same.
If you have multiple Profiles, then Thunderbird and Firefox will only migrate the default Profile. However, there is a way to work around this.
Pre-installation preparation would include:
Empty the Trash Folder in the Mail program
Compact your Mail folders
Delete the Cache Folder, history.dat, and XUL.mfl files
Make a backup copy of your Profiles folder
Leave Netscape 7.2 installed until after you have installed whichever program(s) you decide on, and until you are certain all data has been migrated successfully.
If you are using more than one profile Firefox/Thunderbird will ask you which profile you want to migrate settings from, the first time you start the program, using the newer profile folder name. Firefox/Thunderbird can only import from one source.
You can import one profile, run Firefox with the profile manager, and delete the profile,
but not the files, when prompted to Delete or Don't Delete, then run Firefox again and import the other profile(s).
Run Firefox with the profile manager and create a new profile in the same location as the deleted one and all the settings in it will be picked up.
Follow this procedure for all Profiles, and once they are all in place, remember to recreate the Profiles in Profile Manager, using the exact same name as that which you deleted.
For SeaMonkey, unless you intend to run both SeaMonkey and Netscape:
Make a backup copy of your plugins folder located here:
C:\Program Files\Netscape\
plugins
Make a backup copy of your Profiles folder
Uninstall Netscape 7.2 using Add/Remove
Delete the Netscape Program Files located here:
C:\Program Files\
Netscape
REBOOT THE PC
Install SeaMonkey
When the browser starts, it will automatically use the Netscape Profile
If you plan on running both SeaMonkey and Netscape, then create a new Profile for SeaMonkey
before installation. Sharing a single profile between the two applications will cause corruption.
This is a lot of information, so if you have questions, feel free to ask...
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2) Gecko/20070219 Firefox/2.0.0.2