Your first example (backspace going to previous pages in Firefox on all platforms) has nothing to do with Windows - you could design Firefox not to do this, and I'm sure a user can change this behaviour, or at least an extension.
The sheer volume of users on Windows can be beneficial for OSS software, by stimulating bug fixes and features, as well as providing people to support the software.
As a co-developer of TWiki (before the recent fork by most of the developers, i.e. http://foswiki.org/ ), I supported it on Windows for a few years, and used it myself on Windows for a time when I didn't have a Linux server available. This helped Linux users, e.g. I18N hacks that I did to work around Windows Perl's (very broken) locale support also worked for Linux platforms with broken locales, Perl 5.005 hosting users where I18N regexes were more basic, etc.
Where Windows is somewhat broken but you still have ported the app, you can also deliver a Linux virtual machine using VMware, which is what the TWiki team did as well - this ends up being a good advert for Linux as well.
Doing cross-platform code always risks introduction of bugs, so wherever possible it's best to use a third party portability library, or a portable language such as Perl, Python, etc.