Browse to https://www.whatismybrowser.com/guides/the-latest-user-agent... and copy the latest UserAgent string for Google Chrome.

In Firefox, about:config > general.useragent.override > new string, click +, paste in the value from the website above, click the checkmark.

This will work most of the time on the sites that hired lazy, incompetent web developers to design their pages -- washingtonpost.com, lowes.com, and the worst offender of all, homedepot.com.