Hallo Blacky,
ich hatte das gleiche Problem, und nach mehreren Wochen bin ich in der PDF-XChange-Hilfedatei fündig geworden:
3.1.4. PDFs won't open in Firefox
If Firefox won’t open 'local' PDF's but offers to download ...
Try this :
1) Open Firefox
2) In the Address Bar type in "about:config" and press enter
3) In the "Filter" field type in "plugin"
4) Look for a line that reads "plugin.disable_full_page_plugin_for_types"
5) If this line exists, right-click on it and select "Reset", or just edit the value to remove "application/pdf", leaving the
remaining stuff in place.
If that line doesn't exist, then obviously this fix won't help.
viel Erfolg!