When in e.g. Chrome and Firefox, I am able to drag a tab to a new window. Is there any option to enable the same functionality in Foxit PDF reader?
Unfortunately, I don't think so. I experimented with FR 6.0.6.0722.
I suspect that this simply isn't functionality they ever considered. All tab interfaces are not created equal. Web browsers tabs have matured a lot and browsers have switched to multi-process models, where each tab is an independent process, which lends itself to the "detachable tab" interface. Foxit runs everything in a single process, so the tabs are really just an aid for opening multiple pdf files, not an instance of the process as it is with browsers.