I like this. Simply switch zoom mode. So I set a Firefox original scale toolbar button beside the button this provides. This is enough. Even complex websites have no side effects.

Absolutely essential addon if you hate small text, but get problems in various websites where the entire page does not show, or the page is too wide to fit your screen. It's especially handy when viewing data columns, like on Wikipedia, and the page won't show the entire column because your font is too large. Flip this button and suddenly you can zoom the text in the column. Flip it back when you need to without going into the Settings panel.