Built-in style sheets are never modified if you apply new style rules or make changes to existing rules, either via the built-in Inspector or via Firebug. Such changes are kept in memory although the built-in Inspector (Style Editor) and probably Firebug as well can save the changes that you make to an embedded style sheet to disk. You would have to load such a style sheet manually or add the code to the userContent.css file as site depended rules (@-moz-document domain(<domain>){}) to have them applied on the next visits.

Thank you for your response. I just need a bit more clarification. I've created a page with no author stylesheet therefore the page is being styled using the default html.css. If, for example, I change the font size on my computer to 32px via Tools>Options>General - that stays changed for every visit thereafter and for every site I visit thereafter (where the text is sized using a relative unit of measure).

What I would like to know is where is that local change being saved - is it being saved as a style for p or body elements in a different css file? It does not appear in the html.css stylesheet but it must be save somewhere. My understanding, therefore, is that my unstyled page is being styled by html.css plus a local styled generated through options. Sorry, I know I'm being pedantic on this - I've developed an understanding block on this.

Hi GeorgeCB, to answer your question about where settings are saved, most of the changes you make in the Options dialog will be saved in the prefs.js file in your active Firefox profile folder. You can interactively view/reset/modify them in the about:config preferences editor.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste font and pause while the list is filtered.

Obviously there is a lot of other stuff in here... and the names are not always obvious.