The main question is what behavior is being introduced. I haven't researched deeply, but apparently the add-on does nothing until the user opts-in on studies.
Sorry, but I'm uninstalling firefox. They have broken the basic trust I have in them as a user to not push arbitrary code to my machine against my interests.
Well maybe Safari, not because Apple wouldn't, but because they just don't care enough about ad revenue.
Chrome: They leech everything they can get away with, granted it goes only to Google, but you know it's just to feed their never-ending ad-revenue goal.
MS: They bypassed IE only ads, and went on to build ads into the entire OS.
I happen to like text-only browsers for viewing HTML (e.g HTML tables), tcpclients like netcat for making TCP connections, and my own software for generating HTTP requests. Almost all websites work[FN1], with zero "loading time" as one may experience when using "modern" browsers to do these tasks. I can easily get the content I want (text, with option to download images, PDF, video, etc.) and skip the stuff I dont want. No autoloading of resources. I choose what I want.
Surprisingly, the web is actually getting more, not less text-friendly. Today I can often get text encapsulated in JSON, Markdown, etc. instead of wrapped in HTML, making parsing even easier.
There is heaps of Javascript written by others available on the web today but as a user I have little interest in running it. I would rather write my own.
FN1. "work" means I get the body the page that contains the content.
[1]: http://weboob.org/
I'm not sure I agree the web is getting more text-friendly.. it is getting more JSON friendly, mostly, but actually visiting web-pages where JS isn't required is becoming increasingly rare. I've yet to find a text/console browser that can actually run JS. (I know there have been some experiments, but none that actually work last I checked)..
Do I need javascript to accomplish any of these tasks? No. I am not interested in graphical web pages. I am interested in retrieving information, images, documents, sound files, video files, etc. I never need javascript to get these files, nor to read, edit or view them.
Simple example: I do not need a webpage full of javascript to view a YouTube video. I only need a video player (compiled without networking code).
Years ago, links added a javascript engine. Later it was removed. There is no point. A script can still consume inordinate amounts of RAM, even when there is no GUI. And for what? What is that javascript doing?
In the distant past, I recall browsers used to hand off media files to other programs, based on Content-Type (see article on MIME posted earlier today). Today, these external programs have been subsumed by the "modern" browser.
Perhaps modern browsers can be useful as offline image viewers, document viewers and media players. As I am in text-mode, the graphical browser is on another computer, connected via crossover cable or LAN. After inspecting their contents in text-mode, I transfer the documents and media files to a fileserver.
The decision to forgo using the so-called modern browser comes down to how important web design is to the user. If the user is interested in how fonts look, how a page of text is arranged, icons, styles, etc. and wants each and every website to look different, then the "modern" browser may be unavoidable. Probably javascript is needed to share in the web designers "vision".
The text-only browser OTOH makes all websites look more or less the same, regular, and if in text mode there is only one font, easy to read, IMO less eye strain on black background. It is perhaps better suited for the user that wants fast information retrieval, reliable, efficient file retrieval and cares little about graphical web design.