Where do the ads appear?

Were you previously using an ad-blocking extension such as Adblock Plus or uBlock?

If you don't mind, could you take a look at your extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then on the right side:

(1) Cast a critical eye over the list in case some malware has crept in. All extensions are optional, none come with Firefox, so disable or remove anything unknown or nonessential.

(2) Disabled extensions cluster at the bottom, so check for any disabled ad blocker there.

It's also possible you got adware from somewhere.

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

Adblock Plus Pop-up Addon {web link} Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.

Forum; Adblock Plus Homepage {web link}