JavaScript is very likely already enabled, else you wouldn't be able to do much on most web sites. To check anyway:

Type about:config in the address bar > press Enter

- click 'Accept the risk and continue' if that appears

- search for javascript.enabled

The value should be true.

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See if clearing cache helps:

- press Ctrl-Shift-Delete (Mac: Cmd-Shift-Delete)

- set 'Time range...' to 'Everything'

- untick all items except 'Temporary cached files and pages'

- clear, then restart FF

Also, while on the login page, try clicking the padlock to the left of the address bar > Clear cookies and site data > refresh the tab.

I have to update my Mozilla browser by adding JavaScript to the browser. Why is this required and is there any means to access Adobe using Firefox without complicating the browser by adding JavaScript?

JavaScript is built in as there are no Extensions or Plugins or such to add JavaScript.

Also perhaps you have one or more Extensions installed in Firefox that can affect how JavaScript runs in Firefox, like for example NoScript