Hello,
the title says it all. I can use either a PC or a Mac.
Regards,
-jj. 
On The PC:
I have Firefox 2, 3.5, 3.6 installed as portable version and Firefox 4 installed for real. I also have an old version of sea monkey (1.1.17) which renders much the same as Firefox 2. Plus a Netscape 9 version although I rarely check in that.
I have multiple versions of opera installed but usually only test in the last couple of versions.
I usually only keep the latest versions of Safari and Chrome as they are changing too frequently to keep track of.
For IE I use Iester for ie6 - 8 while runing IE8 for real on my main PC (XP). I also have a laptop running vista which has IE8 running plus ietester installed.
On My mac I have The latest versions of Safari, Opera, Firefox etc. And then under parallels on the Mac I have Windows 7 installed running IE9.
For most browsers apart form IE you can download the older versions anyway so it’s not that great a problem but I tend to test in modern browsers while developing plus always checking in ie6 - 9 at frequent stages.
I prefer the Mac option (well, OK, I’m on a Mac anyway) because I can run everything there. Instead of Parallels, I use VMWare Fusion, which I really like, though I would probably have used VirtualBox (a free alternative) if I’d known about it. I haven’t tried all those options for multiple versions of browsers, but rather have different versions of each in various virtual machines. Probably overkill, but it works for me.
Most of the “tools” like IEtester or MultipleIE do not behave like the real browsers in terms of things like javascript and even some CSS elements – likewise FF4 will refuse to start if 3.x is already running… and online tools like “browsershots” are effectively useless for real-time checking of code changes AND are completely useless for checking things like hover states… and most of the time bear no resemblance to the actual browsers in their native environments.
Which is why I use VM’s under Oracle VirtualBox. If you can’t find a dead machine with a XP license sticker on it, and a XP CD there’s something wrong. Put in a bunch of separate installs each with their own version of IE and FF.
There is NOTHING like testing in the real operating environment as opposed to broken/buggy tools that really don’t get the job done right.
With regards to CSS I find ietester renders perfectly and have yet to find a situation on my machine where it didn’t behave as it should do (although I think I did see something different once but can’t remember what it was). I use it hundreds of times a day for the last few years and never really have a problem with it apart from it crashing regularly - which is a pain.
I will admit that others have said they have seen problems but I guess it depends on the exact set up. (I wouldn’t use it for JS testing though.)
There is of course no substitute for running the native version if you can however and even virtual box set ups seem to offer different results on occasions. Having a single computer running the single version/OS is the best solution but you end up with a desk full of old computers. I did keep a couple of old lap tops for this purpose but it just wasn’t worth the effort when I could run ietester in a second.
It seems nothing is perfect or representative of all machines. You would need to have all the different OS running all the different browser versions for perfect testing which is taking things to the extreme.
likewise FF4 will refuse to start if 3.x is already running…
I use the portable versions but they suffer from the same problem in that you can only one run at a time unfortunately. (That’s why previously I installed an old sea monkey which renders the same as FF2 and allows me to leave it running.)
Sihhh… I guess I’ll have to set up virtual box so I can test ie9. What a pain.
You can run different FF versions side by side on windows. You just need to make sure each instance runs on its own profile. That sounds more difficult than it is, see DouZeWorld » Using Firefox 3 and Firefox 4 side by side
Most likely culprit of that is usually failing to test for unusual differences – there were no less than 52 separate patches to IE6 alone, many of which changed it’s behavior. As the trident engine is also used to render the UI, different versions of the OS can also have a great impact. Generally IE 5 and IE6 both behave differently under 2k and XP than they do 9x… Usually I’ve found if it works in 5.5 under 9x, you’re good to go in 2K/newer with 6. Which is why I still test 5.5 and 6 under 9x instead of XP… XP I use for IE 7 and 8, with 9 installed native under Win7.
I don’t make it “perfect” for them, but the page should at least still be usable. (even if the layout has some issues like some elements not centering).
Though my simple rules of “never delcare width or height on same elements as padding/border, don’t trust the default line-heights, if you change font-size redeclare the line-height, put haslayout triggers on the parents of positioned elements” is why I can still write pages that work just fine on the latest bleeding edge while working just fine all the way back to IE 5.5 (and sometimes 5.01)
I mean, apart from once in a blue moon needing zoomfix… that’s it. I almost wonder why people think supporting the older IE versions is “hard” – then I see their code and go “oh…” Well, that and there’s a lot of stuff I wouldn’t do in a layout in the first place; typically the stuff that leaves the art major going “but I can do it in photoshop”
Do you have a blog, or a ressource where you share your thoughts in more depths and/or give advice as to what to take into account to design for older browsers?