The firewall and antivirus will work, it is the add-ons that are not compatible. Symantec will need to update their Firefox add-ons so that they are compatible with Firefox 4. They have indicated that for Norton 360 they plan to release an update to Norton 360 to support Firefox 4 in early May. Pending the update by Symantec, if you want to use the Norton add-ons you will need to downgrade to Firefox 3.6. To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data. You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc. To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations. For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

Update: Symantec have released an update for Firefox 4 compatibility, for details see http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=20100720113635EN&ln=en_US