You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Load the web page. After the page is loaded, click the down arrow next to the AdBlock icon. Now select Disable on <web site>.

Forum; Adblock Plus Forum {web link} https://adblockplus.org/ Homepage

Using Internet Explorer, open this link for AdBlock for IE. https://adblockplus.org/

You should be able to make site-specific exceptions for pop-ups. You can use the Permissions panel of the Page Info dialog while you're on your school site(s). You can call that up using any of these:

• right-click and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or globe icon to the left of the site address > More Information > Permissions

Under Open pop-up windows, click Allow. Once you have allowed the site that need to be allowed, you can turn the pop-up blocker back on on the Options page.