If you can afford in your project, use a external service that tests with people with different disabilities.
[1] https://webaim.org/projects/screenreadersurvey7/#browsercomb...
It's worth keeping a few points in mind when reading these results:
1. The survey was carried out in October 2017. Things can and do move quite rapidly in this area, especially given the increased number of users who will be running Windows 10 now who may not have been back then.
2. IE won't be around forever, and once the accessibility of Microsoft Edge improves (which is likely to happen pretty soon when they adopt Chromium), IE will become less and less relevant.
3. NVDA with Firefox is a much more common combination than JAWS with Firefox, primarily because the NVDA developers recommend it. Freedom Scientific (makers of JAWS) have historically pushed IE perhaps more than other browsers, so it will be interesting to see how that changes as Edge becomes more usable.
I'm well aware that these combinations will change over time.
The biggest thing is that JAWS costs a lot of money, so people buy, get used to it, and stick with their setup.
I tend to say, 'The only people who still use IE have no choice" or something like that.
I'm not sure that's strictly true nor measurable, or really matters in this case when we're discussing a survey filled out by users.