When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.
Be sure to let me know that the error is an HTTP 403 error and what steps, if any, you've already taken to fix the problem.
When the Installation finishes, it will attempt to invoke the Turbine Client Launcher. This will fail and (should) pop-up an error window. In the CXG installer window, the "Downloading Lord of the Rings Online" should be the only item running.
All of the other entries should show "finished." To the right of the "barber pole" and the word "running" is an "X". Re-Launch Cross Over Games from the Applications directory /Applications/Cross Over Once Py Lot RO launches, use the "tools" pulldown, "Settings WIzard." Select "Lord of the Rings Online," click "Find Games." Select the "C:\Program Files\Turbine\The lord of the Rings Online" entry; click "apply." In the "tools" pulldown, select "patch;" click "start." You should see something like: At which point, depending on the size of the patch and your line speed, go get some cofee, etc.
(You also can clear Firefox's cache completely using: orange Firefox button ''or'' Tools menu No Proxy (2) Have you checked for malware?
These two free tools are highly regarded for supplemental scans: Malwarebytes Anti-malware : SUPERAnti Spyware : (3) Did you try Firefox's Reset feature?