Next time the item becomes red, please hover the mouse on the red cross icon, and report the error message shown. This may help me know the reason.

The cache directory is %APPDATA%\Mozilla\Firefox\Profiles\***\storage\permanent\moz-extension+++***\idb\***.files\ . The extension should move the file out when the download is finished.

About the HTTPS problem: I may be able to workaround by prompting the user. However as far as I know, most HTTPS sites are trusted by the browser and do not need manual approval. If all HTTPS websites show the certificate prompt, there is a possibility that your ISP is actively monitoring your Internet connection.