I heard one can load a custom Firefox profile when starting webdriver, but I've yet to find a way to do that. The Python binding documentation doesn't state it very clearly.
I need to start up Firefox without JS because the site I'm testing has a lot of ads injected by Google and some are very slow to load, making the tests slow as well because it waits for all the page objects to finish loading.
You can use the -firefoxProfileTemplate command line option when starting the Selenium server. But it seems rather counterproductive to disable javascript when testing how browsers behave on your site (unless your site doesn't have any scripts of its own) - you should rather use adblock, or disable the IP used by Google ads in the hosts file of the Selenium server, or set a custom useragent for Selenium tests and don't load ads based on that.
How to load Firefox with javascript disable when running Selenium Webdriver (Python) tests?