I think you probably have no choice but to pay for a later version. I've been there myself
Gecko support was introduced in v8, and it looks as if it uses its own modified version of the display engine rather than the one which comes with Firefox. However as V8 and above support current versions of IE you probably wouldn't want to bother with Gecko.
Re Internet Explorer 8 - which is the version I think you need for old versions of ABCpdf - if your server came with IE8 installed to begin with and it's been automatically updated, then it should in theory be possible to roll back to the previous version, or even rebuild the server from scratch, and then block windows update from updating IE. If it came with IE9 or above then you really do need to upgrade ABCpdf
Aha, didn't realise that this feature was added in v8+, and didn't realise I didn't have to install FF. I think you are right - new version of ABCPDF is the only choice.