A few things that I find a pain in Firefox on Ubuntu (coming from using Windows at work everyday)

Navigate to about:config by typing it in the URL bar, read the warning and continue. Search for the settings and change as appropriate.

UK English spellchecker:

spellchecker.dictionary = en-GB

Mouse scrolling action

Feels a bit weird coming from Windows, these three setting keep it more in keeping with the Windows scrolling experience:

mousewheel.system_scroll_override_on_root_content.enabled;true
mousewheel.acceleration.factor;2
general.autoScroll;true

URL bar autoselect

This one selects all the text in the URL bar with a single click: browser.urlbar.clickSelectsAll;true

Dark Theme

Easily enabled - I think this takes the setting from the dark theme being enabled in Ubuntu. If not, enable Dark Theme in the menu ☰, Addons, Themes. There should be a Dark Theme that's been created by Mozilla.

When also running Ubuntu in a dark theme, the textbox and other form elements within Firefox inherit this dark theme, even though they shouldn't (as the page you are viewing may be white). This can be overridden with this additional string setting (websites with css customised form elements aren't affected): widget.content.gtk-theme-override = Adwaita:light [this setting might not be necessary anymore]

Hiding the titlebar

Hide the titlebar, as it's not really necessary; go to ☰, then Customize, then at the bottom there is a tick box to enable/disable the title bar.