To answer a few questions
The sudden upsurge in version numbers came about as an attempt to "keep up with chrome". Google used a stupid release numbering system, with each trivial update getting a new version number - so the powers-that-be at Mozilla decided they had to follow the trend or be perceived as being left behind....stupid
As to vulnerability to hacking - the newer versions are much more resistant to known attack vectors. Obviously the crooks are constantly coming out with new hacks, and - the browser will always be one step behind, but the newer the version, the more secure you are.
You can of course increase the security of Firefox by using a bunch of security plugins:. If you use ALL the following: AdBlock Plus, AdBlock Plus popup blocker, NoScript, Ghostery, BetterPrivacy, HTTPS-Everywhere, BrowseerProtect.org along with a decent antivirus program then you've made Firefox close to bulletproof.
Out of the box, none of Opera, Chrome or Safari are as secure as either IE or Firefox, though there is a No-Script plugin for Chrome which helps a bit. Safari is incredibly vulnerable - don't even think of using it. The open-source engine of Safari is as buggy as hell.
TZ350
when I say "hacked" I mean you'll end up with malware of some kind on the machine. Could be anything of keyloggers, trojans, worms or numerous other hazards. They can steal your credentials, you bank details, redirect your browsing, or even rent out processing tim on your computer to third parties
As to you hotmail problem - no that was due to a password hack. A few weeks ago a large number of Hotmail accounts were compromised because somehow the passwords were cracked. You need to change the password to something unguessable, and also make sure the security reset questions are unguessable as well.
No A/V program on your PC would have stopped that Hotmail hack - its almost certainly caused by a weak password. However, your choice of antivirus program isn't great: Microsoft Security Essentials is a bit "slow" at identifying new risks, and also not all that comprehensive in what it does. Its really only useful as a compromise product on older slower machines. If you want a free A/V program that works, then use Avast - it consistently has a malware identification hit rate of around 95% (which puts it in the top two or three programs). If you also install Threatfire then you'vw got reasonable security. The other high-scoring free A/V product is Avira, though that is not as comprehensive in what it does