I'm using this addon for years already and it always worked as expected, until the latest update for Quantum.
Ever since it breaks the website of my email provider www.Web.de and only deactivating FoxClocks helps.
Hopefully this can be fixed in a next release.

Besides that a suggestion: an analog or digital clock as icon would serve the purpose to have a visible clock in firefox, without the need to click somewhere or have a full toolbar always visible.