Let's do a few things to improve your situation here:

• First, update to Firefox 38.0.1 to increase your stability, performance and security. Update Firefox to the latest release
• Then, scan for malware (which can often give you more ads). Troubleshoot Firefox issues caused by malware
• Finally, install www.adblockplus.org

I totally agree with investigating potential malware (including sketchy extensions), especially if ads are appearing in unexpected places. Assuming that checks out clean...

By default, all sites are allowed to use Flash. However, you can take control of site permissions for the Flash plugin and only approve it where it's actually valuable by using the click-to-play feature.

To set the permission:

Open the Add-ons page using either:

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

In the left column, click Plugins. On the right side, look for "Shockwave Flash" and change "Always Activate" to "Ask to Activate".

To refine the feature:

There is an add-on which changes activation from "all or nothing" for a site to player-by-player. If you find that you need that extra level of control, you can read more about it here:

https://addons.mozilla.org/firefox/addon/click-to-play-per-element/

You may want to look into uBlock as an ad-blocker. https://addons.mozilla.org/en-US/firefox/addon/ublock/

IMO, AdBlock Plus has gotten to be too much of a CPU hog. And it has too many feature that "normal" users don't really need. I just with the guys over at the Add-ons website would get around to doing a full review of uBlock.

As far as blocking Flash from running at-will, try using Flash Control. https://addons.mozilla.org/en-US/firefox/addon/flash-control/