What do you mean; Flash Player popup? Are you talking about a pop-up / ad?

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.

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}

The text on Firefox is very faint and unreadable

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

It appears that your Firefox currently is set to run in Windows 2000 compatibility mode, which may break some functionality of Firefox. Could you turn off the compatibility mode? Right-click your desktop shortcut for Firefox, choose Properties, then the Compatibility tab. Please uncheck that box and anything else selected on the tab so that Firefox runs with the full native functionality of whatever version of Windows you actually have.