How to bystep a websites ridiculous javascript enforced browser checks?

How to bystep a websites ridiculous javascript enforced browser checks?

I have a social problem that I want to find a technical solution to. My
employer uses a website for HR management that insists on me using a
Windows, OS X, or iPad browser. This, of course, is not something I'm
willing to do, as my platform of choice is a GNU/Linux system.
The offending site in question is: http://on11.ultipro.com/XpressHR/
At first, I thought that a simple change of user agent would suffice, but
it turned out that the page uses javascript to check not only for the
browser, but addons too, e.g., when I tried using Firefox under Wine, it
complained about missing Adobe Reader. Wine and Firefox on Windows are at
least free software, but I'm not going to install the proprietary Adobe
Reader to satisfy this site if I can possiby avoid it.
My next attempt has been trying to use Firebug or the Firefox developer
tools to stop the javascript and at a convenient point change the
javascript variable "browserIssueFound" to false. But here I've hit a dead
end so far, I haven't figured out how to get the debugging tools to work.
The assumption everywhere seems to be of debugging my own code, not a
hostile code from some website. The break point capabilities usage
instruction is to set them and then reload, but this doesn't work for me,
as the script rewrites the location field (and thus redirects) to a static
page before I can do anything.
Any help with my proposed solution or different solutions alltogether are
much appreciated. Turning off javascript doesn't help, as the forms rely
on javascript for basic functionality.