System Restore doesn't affect some things like your "My Documents" folder and such, but you can always try.. Yes, if you use System Restore, it's possible you'll need to renew the subscription depending on the type of update the program uses. Some subscription renewals are actually stored on the computer itself while others are managed from the subscription server.
That said, why not simply "import" the Bookmarks from your Internet Explorer Favorites section. In Firefox, click on "Bookmarks" in the upper left, choose "Manage Bookmarks". When that window opens, click on "File" in the upper right, choose "Import", make sure the "Internet Explorer" box is CHECKED and then follow the prompts to import Internet Explorer's Favorites to your bookmarks.
Hope this helps.