Look into running separate sessions of your browser(s). Both Firefox and Google Chrome (or Chromium) allow you to do this, although the interfaces for doing so differ.
A simple way to do this is to use different brands of browser, e.g. Gmail in Chrome and everything else in Firefox. But... if you really prefer one browser over another, for all use, then the separate profiles thing works.
Note that in Chrome, this is now confused by the ability to change Google account log-ins. That is not a separate, browser-level profile with separate configuration.
Instead, you are looking for the command line invocation argument --user-data-dir (in *NIX, at least; IIRC the flag name may differ slightly in the Windows version).
For Firefox, there is the -p flag. IIRC, you have to combine it with another flag in order to ensure both that the profiles are running in separate invocations and that you can be prompted to choose what profile to use when you invoke Firefox.
Of course, you can create menu items / icons for these invocations to make them "clicky" and avoid having to go to the command line and enter them each time, if you prefer.
P.S. Yes, this will help you less if you insist upon clicking directly on/through links that are are e.g. mailed to you or, if you have Facebook in its own "box", posted on Facebook.
From that perspective, having per site browser extension variability might still be useful. But then, you're still looking at also controlling referer passing, cookies and other local data, etc., etc.
To run a new, separated Firefox with a (possibly) different profile
firefox -no-remote -ProfileManager
It's always handy to have a "vanilla" profile, to compare how much the extensions tuned down the browser or try to understand if the error that you're seeing is caused by an extension. Having a "privacy" profile with some ad-hoc extensions helps too.
Mind you, -ProfileManager actually opens to the full profile manager interface (where you select a profile to run, or create a new one, or whatever). You can load a specific profile (that already exists) directly by replacing "-ProfileManager" with "-P [profile name]". (Omitting the name will open the manager, too.)
Thanks. Sorry I mis-remembered the flag(s) from memory. And be sure you're using both, to make sure the separate profiles do not share the same process or something like that (again, from memory; Google can quickly turn up the details).
Your memory is right. Without -no-remote you would end up spawning another windows from the currently running firefox. Without -ProfileManager you can't choose a different profile.
It's also a good idea to use different themes per profile (and I see you suggested it too).
I tried running a Chromium session with no extensions for the stuff I want to be more secure (email/banking/etc.), while using Chrome for everything else, and this does work in a way. But I find that I tend to forget to switch to the other browser sometimes. A solution which said "don't run extensions on mail.google.com and online.my_bank.com" would be a lot more convenient.
The separate browser solution does protect from tracking, but with the security threats of today, like these malicious extensions with access to all your data, I've become desensitized to mere tracking.
I changed the color scheme for one profile (although, that involved installing and trusting the color scheme; I got mine directly from Google's site as opposed to a third party site).
An extra cue, when the border background, tabs, etc. look different in one versus the other. Still hardly foolproof...
A simple way to do this is to use different brands of browser, e.g. Gmail in Chrome and everything else in Firefox. But... if you really prefer one browser over another, for all use, then the separate profiles thing works.
Note that in Chrome, this is now confused by the ability to change Google account log-ins. That is not a separate, browser-level profile with separate configuration.
Instead, you are looking for the command line invocation argument --user-data-dir (in *NIX, at least; IIRC the flag name may differ slightly in the Windows version).
For Firefox, there is the -p flag. IIRC, you have to combine it with another flag in order to ensure both that the profiles are running in separate invocations and that you can be prompted to choose what profile to use when you invoke Firefox.
Of course, you can create menu items / icons for these invocations to make them "clicky" and avoid having to go to the command line and enter them each time, if you prefer.
P.S. Yes, this will help you less if you insist upon clicking directly on/through links that are are e.g. mailed to you or, if you have Facebook in its own "box", posted on Facebook.
From that perspective, having per site browser extension variability might still be useful. But then, you're still looking at also controlling referer passing, cookies and other local data, etc., etc.