I posted a similar inquiry in a different thread in the Themes forum, but it hasn't gotten any attention there at all. So I'm trying it here..

Trying to activate a new theme will result in an infinite loop when you visit the website after changing the config.php. You can't change it back to the old theme and have the site working normally again.

The only fix I've found is to completely wipe the site and start over again with a fresh install. Then of course, we're back at square one when trying to switch to a new theme.

BTW... obligatory info:

Linux CentOS on a dedicated server
SS 2.4 beta / 2.3.6 ( I tried both versions )

What do you mean by infinite loop? Is your page timing out, or do you mean a redirect loop?

The only reason I can think of that changing your theme breaks anything is if your manifest is not be regenerated properly. After changing the theme, empty your webserver tmp folder for SilverStripe (usually named something like silverstripe-cache, I think) and see if that helps.

I mean Firefox detects that the page is loading in a way that'll never complete, and it halts the loading process.

Could you post the error you are receiving. It is more likely that PHP will stop execution before FireFox times out.

Did you try my suggestion?