Of course it could be costly (in time mostly). But eventually, everything reaches end-of-life. Windows XP, IE6, IE10 will too...

For years, organizations said switching to Linux is more costly than paying the MS tax. Recently, many large organizations rolled out Linux installs on a massive scale (European government institutions).

Anyhow, installing Firefox on some workstations is relatively small compared to say, replacing COBOL, or Windows XP...

Edit - and while I don't manage a large computer install base, I use them at a large organization with thousands of installs (a University), and they somehow managed to install Firefox on every single computer.... (but they still advertise COBOL jobs)