I doubt it.
The problem is that there is a significant population of people who hate Microsoft and want to hurt the company, and their products are complicated and created by large teams.
There will always be bugs, regression errors (where a bug that was fixed in one release is not in a subsequent release, due to the bug being added to one development code and that not being used for the newer release, usually due to simultaneous development by more than one team) and general logic flaws (compromises) in the designs.
IE 7 is playing catch-up to Firefox, but should be equivalent in features (Opera still has better features that the others are implementing catch-up); I would predict that IE 7 will not be fall behind Firefox by as much (if at all) again, because M$ is now prioritizing it (and must have some spare capacity when Vista rolls out the door).
Firefox is regarded as the underdog, so it doesn't attract every script-kiddie trying to make a name for themselves / stick it to da man, though there are viruses and trojans for EVERY OS, including Linux and OS X.