Currently, Firefox does not honour the Parental Controls you've set up in Windows Vista - there's currently a bug in BugZilla on it.
Personally, I find some of the comments on that bug kind of funny. Some people seem to be morally opposed to Parental Controls, which I don't quite understand.
I don't understand why this "bug" needs to be addressed anyway. I understand that Firefox wants to be universally accepted, but it's not like you guys are stuck doing exactly what Microsoft wants.
Last I knew, Firefox wasn't owned by Microsoft, nor was it licensed to do everything in accordance with Microsoft's standards and security features.
Which is fair enough, but if you don't like parental controls, you don't need to turn them on. I imagine for a number of users, however, parental controls are a great feature. I'm not saying it's going to stop your 16-year-old computer-whiz son from downloading, e-hm unsavory images (that's what adequate supervision is for) but it'll certainly stop your young kids from accidentally navigating away from neopets.com or something.
Others believed that there's no point in implementing the feature because you can get around it anyway.
Trying to block downloads in firefox wouldn't really help anything with circumventing restrictions. If the user can view pages then he/she can pretty much download any file they want. Just blocking the download manager wouldn't help anything, one would just use a seperate download manager to download the files(wget or something like that would suffice and wouldn't need to be installed so even with all the restrictions a user would be able to use it)
But that ignores the fact that in order to use wget, you, um, need to download it in the first place...
I guess it's theoretically possible to download it onto a USB key from a non-parental-controlled computer (your mate's computer, for example) and run it from there on your parental-controlled computer, but that's assuming that the parent disabled downloading, but did not disable running arbitrary programs (which is like locking the front door while leaving a window open). Perhaps it was just a misunderstanding on the part of the commentor as to what parental controls actually lets you do.
My point, I guess, is that Firefox should just implement parental controls. There's really no reason why not (after all, if you really are morally opposed to the feature, just don't turn it on in the OS). Luckily, it seems, the detractors are in the minority and it's only inevitable that Firefox will have them. One of the good things about open source software is that it only takes one person to implement a feature, but it takes consensus to not implement one. And OSS is not known for reaching consensus very often :)