Sounds like a Windows shop. They don't know Linux. They don't know how
to handle the UA string that reports the client is coming from Linux.
Can you use their site if you use an add-on to send a UA string that
makes you look like you are using Firefox (or even IE) on a Win7 host?
What UA string is your Firefox instance sending? There are sites to
tell you what they got from your client (
http://useragentstring.com/).
I've seen sites that use a table of allowed versions of a web browser
but they are stupid in setting an upper bound. That is, they check for
a max version but it is less than the currently released version so they
block with a bogus error that your web browser doesn't meet their
criteria for client version. You might have to set the UA string to
report an earlier version of your client.
Since this is a gov't site, it is no suprise they are using the ancient
method of using the User Agent string to determine the capabilities of a
visiting client. The UA string can be spoofed (and, as such, can be a
solution to their defective detection) and doesn't really relay what a
client can support. Using the UA string has been long considered a bad
or limp method of detecting client capabilities but many sites still do
it. window.navigator.userAgent is deprecated and should not be used.
The recommended method is feature detection.
You will have to wait until the site gets around to updating their table
of UA strings to include the one reported by your client, or you could
use a UA add-on to lie to the site.