What is best-practice way of consuming a SOAP web service in Firefox OS?
Are there any Firefox OS specific issues I should pay attention to or it comes down to a problem of consuming a web service from JavaScript (where I can use JQuery AJAX or similar)?
There shouldn't be any Firefox OS-specific issues - if/when you could make it work in a browser (preferably a Firefox browser, that is) then it should work just fine in Firefox OS, too.
Although one issue might arise, if the SOAP service doesn't serve CORS allow-origin headers, in which case your XMLHttpRequest would fail as the OS won't allow cross-origin XHR-s. You could overcome this by creating a Privileged application, that requests in the webapp manifest the SystemXHR permission, in which case you are allowed to make cross-origin requests to remote services, regardless of CORS headers.