General :: How To Change Firmware But Not To Lose Possibility To Update It

As far as I know, if an owner of an android smartphone decides to flash/update his phone by not using the official ways (by smartphone via wi-fi or by pc via Kies), then he permanently loses a possibility to update his phone officially.

I know that because I used to have Galaxy S2 and installed a custom firmware, although there was a trick when you use some stick to have your phone "official" again.

So, I'm not comfortable with the "latest" android OS 2.3.6, S6802XXLF2 / Gingerbread.XXLF4 My phone is GT-S6802 Ace Duos. I'm not comfortable due to the fact that the default browser doesn't have an exit button, so I have to resort to task manager in order to kill the process of internet browser - which is an oversight by android devs.

But I know, that in some new firmwares the default internet browser has an exit button, so I'm wondering if there's any firmware to which I can upgrade without losing the possibility to update my smartphone afterwards.

Also, I take it, for every smartphone there's certain firmware version, right?

For example, for Galaxy S3 it's Android 4.0.4

While for Galaxy Ace Duos it's Android 2.3.6

And the OS of Galaxy S3 can't be installed onto Ace Duos, right?

This is sad, why can't Samsung make one universal OS for its smartphones? Does Windows Mobile has the same issue? Or is it universal just like PC Windows OS?