Content
Usually refers to webpages being rendered, may also be seen as "running in content" in which case it can include UI privileged code running in a process that is rendering webpages.
Chrome
Usually the parts of the application UI outside of the rendered webpage
XUL
A proprietary UI definition language that we are slowly phasing out and replacing with standard HTML.
l10n
An abbreviation for Localization.
i18n
An abbreviation for Internationalisation
a11y
An abbreviation for Accessibility