You can downgrade to Firefox 56 and reinstall the 1.2.13 version of Web Developer from here to change any of the old built-in browser settings:

https://addons.vaultpit.io/en-US/firefox/addon/web-developer/versions/

Or you can use about:config to reset the browser.cache.disk.enable, browser.cache.memory.enable and network.dnsCacheExpiration preferences:

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Or you can create a new profile, but that will remove all your settings including bookmarks etc:

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles