I know people who get paid by their friends for help setting up computers - often reformatting/reinstalling after malware infections - and I'd guess many of them would be installing Firefox for them too. Does it mean they can't do that according to Mozilla if they charge for the services? Dell gets picked on here, but I bet they're not the only ones who charge to install free software like Firefox.
It's totally fine for anyone to charge to install Firefox -- the software is open source. They cannot advertise their services as a Mozilla Firefox Install Service -- Mozilla's trademark is not free. At the very least, you would have to be very clear that you are not affiliated with Mozilla in any way.
It's pretty clear (to me - IANAL) that Dell is using Mozilla's trademark without their permission. If they called it Open Source Pre-Install Service, and explained that it includes Mozilla Firefox and some other software, I think it would be fine.
