"A web page is slowing down your browser"

I'm on a Lenovo ThinkPad Edge E560 PC that I've had for four years. I'm still using Windows 7 and conscientiously installing all the updates. It's still working well. I use Firefox as my browser. Lately, whenever I've been on my home page, a banner pops up at the top of the screen that reads "A web page is slowing down your browser. What would you like to do?" and gives me the options "Stop It", "Wait" or X. None of the options work for very long. I found an article on the internet by Karan Khanna that lists four fixes for this problem. I tried the first two - "Clear cookies and site data" and "Update your graphics card driver" - but they didn't fix it. I've hesitated to try either of the last two - "Modify a few settings for Firefox" and "Disable Adobe Flash Protected Mode" because I was afraid of making a serious mistake. However, the problem is getting worse. Khanna also recommends "Download this PC Repair Tool to quickly find & fix Windows errors automatically", but again, that seems risky. If anyone here has a better idea, please let me know before I take it in to the repair shop.