Isn't Firefox mobile able to save tabs when switching to another app and back to Firefox? Besides its slowness, one big problem of Firefox to me is whenever I leave the app (e.g. looking up a translation in a dictionary) and switch back via ALT-TAB or long press HOME, Firefox reloads all tabs, they are already empty when reopening Firefox (so Ff seems to lose the tabs and only saves the URL's). 

Is this due to low memory of Smartphone (500 MB RAM) or does Ff show the same behaviour on Tablets and high-specs smartphones? I also need Ff for Tiddlywiki and switch between apps and reopen Ff quite a lot. So a solution/explanation for this problem would be fine.
Android never removes applications from its memory, unless it has no memory left, then it initiates a low memory warning to unused applications and kills them when needed... 

Android WebViews (The default android implementation of web browser) consume lots of memory, and so is Firefox's implementation of their web browser. Actually, Gecko (Firefox's rendering engine) is much slower and a bigger memory consumer than Android's default Webkit rendering engine.

So when you exit Firefox on your Android, the memory is probably nearly full, the system kills it, but Firefox has a mechanism that saves the URLs of the previously opened tabs, and though it starts the application again (and not resuming it) it loads everything you left open again... 

And no,512MB are no enough, Android 2.3 takes approx 300MB to run.