Even without a Firefox OS smartphone you can now do some testing and debugging of how a website is likely to work on Firefox OS devices, by installing this add-on in Firefox on an Android phone.
Change the add-on's settings to enable the functionality (disabled by default). When the relevant preference is enabled, Firefox on Android or Firefox desktop will pretend to be the Firefox OS web browser when visiting websites. (You will typically have to re-load a website to see the effect, and sometimes you may have to log out/in, start from the site's home page, or delete the site's session cookies).
Many compatibility problems can be discovered through such testing. However, a full emulation of Firefox OS devices' behaviour is not possible - in particular, issues regarding performance and video playback are best tested on actual hardware.