Disabling automatic form autofilling in Firefox (which is now simple)
If you allow Firefox to memorize your logins and passwords on web
sites, by default Firefox will automatically pre-populate login
forms with them. However, for years I've had this turned off,
probably no later than when I read 2017's No boundaries for user
identities: Web trackers exploit browser login managers.
I was going to say that there's no Preferences option to control
this, but it turns out that there is these days, in "Privacy &
Security"'s "Logins and passwords" section as 'autofill logins and
passwords'. This controls the about:config setting
signon.autofillForms. If you untick
this option (or set the value to
false from the default
you need to click or otherwise select the field before you'll get
the option to autofill it.
(In other articles about this general issue, there's 2021's You should turn off autofill in your password manager (via a Firefox issue) and probably others.)
There is a long standing open Firefox bug, bug #1427543 make signon.autofillForms = false the default. The bug doesn't seem to have gotten much commentary or attention over the five years it's existed, so it's not clear if Firefox will ever act to change the default (or definitively reject the idea, for that matter). These days there is a related preference specifically for HTTP login forms, signon.autofillForms.http, which defaults to false (off) now (bug #1217152).
(There's also a separate 'signon.autofillForms.autocompleteOff' preference, which is described in bug #1531135 Add a pref to not autofill in password fields with an autocomplete field name of "off".)
I'm glad that Firefox picked up an explicit Preferences option for this, since it means I don't have to remember (or hunt down) the name of the about:config setting the way I used to back in the days. Admittedly I don't need to do it very often, but I do set up a new Firefox instance or profile every so often and this is one of the changes I want to make.
(Until I did my research for this entry I didn't know about the Preferences option, so I was mostly writing it to capture the name of the about:config setting for future use.)