What are they paid for?
Hey, this is a call center. The guys are supposed to use the application provided by orange and service customers.
They want firefox because they like browsing for personal purpose at the same time. If employees think they would be more productive with enhanced tools, they can argue about it, but they are not allowed to choose themselves what they believe suits them best.
It is perfectly acceptable for a company to have restrictions on the usage of its stuff I think.
Look at other jobs: you are not allowed to tinker your bus if you're a bus driver, you can't hack yourself your IRM scanner when you work at the hospital, you can't "customize" the lifts at your company, even if they are slow.
All this remembers me the "Y2K" projects where the whole IT industry deemed it legitimate to make the customer pay for faulty design. You have to accept that IT software as well as hardware is an investment and it is legimitate for a company to get value from it as long as possible. The "constant upgrade" mantra is just plain robbery.