Does this apply when all the major browsers (save IE, though it's been a while) update unobtrusively?

Yes, it would. It's not based completely on performance gains you may see in new updates. Profiles bloat up over time with data (history, cookies, cache, etc) and the bigger it is, the more the browser has to process. So, over time, your browser will slow down. A fresh install of a new browser would result in all this data not existing. If it doesn't have to parse hundreds of megs of files, it'll generally be faster.

All of those things are easily cleared, however. If I do a clear history/cache/cookies in Firefox, there's still a lot of miscellaneous data floating around in my profile, to the point where, last I recall, the Firefox team still recommends you outright dump your profile in case of difficulties.

I don't see how it has anything to do with how obtrusive the updates are.

you can always go to about:support and click "Reset profile..."

Browsers aren't like Windows 98. You don't need to reformat or defragment from time to time.

Already covered by other commentators, but this is straight up wrong if they are implemented in certain fashions.

Here is a large history causing slow load times; https://bugzilla.mozilla.org/show_bug.cgi?id=223476

Here is their recommendations regarding download history, which I myself have experienced (too many downloads remembered forever slows down the browser) https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-re...

There is nothing inherent in the design of A hypothetical browser that would make it slower over time, its just a matter of people make assumptions and those assumptions are often wrong, and then performance suffers.

Some things do accumulate. That is in fact the point of Firefox Reset[1].

The support page lists "Extensions and themes, website-specific preferences, search engines, download history, DOM storage, security settings, download actions, plugin settings, toolbar customizations, user styles and social features will be removed."

[1]: https://support.mozilla.org/en-US/kb/reset-firefox-easily-fi...

Installing extensions can dramatically slow down browsers. I wonder if the lack of extensions is responsible for the quickness users experience with a fresh install.

Absolutely – there's really common cycle I see repeated all over:

1. “(Firefox|Chrome) is slow” 2. “Did you try removing Ad-Block Plus, etc.?” 3. “Now it's fast again!”

I think we're well past the point where the browsers are going to need to start having some UI around monitoring and exposing slow extensions to make it easier for users to learn this.

Absolutely. When I first starting using FF and all the developer tools, it would grind to a halt. Now that I'm a lot more comfortable with the FF toolset, I have a minimal set of extensions and it feels much snappier.

I also use Aurora which is still super fast to me. I also develop with Chrome Canary and the stable Chrome version. Canary has times when its totally useless or when someone breaks the build and I can't use it for a week so until the next update comes out.

All in all, yes, less overhead means a snappier browser to me.

That's my biggest reason for switching from AdBlock+ to Privoxy + https://github.com/skroll/privoxy-adblock . Same blocking rules but uses a ton less memory, since each tab isn't running all of AB+ .

That's fine for image and flash ads, but provides no support for CSS blocking of elements. My understanding is that the CSS blocking is the slowest part, too.

Of course, if you don't want that, you're sweet.