--- a/browser/components/extensions/ext-utils.js
+++ b/browser/components/extensions/ext-utils.js
@@ -315,17 +315,19 @@ class BasePopup {
       // maximum height, so that the PanelUI resizing code can make an accurate
       // calculation. If we don't do this, the flex sizing logic will prevent us
       // from ever reporting a preferred size smaller than the height currently
       // available to us in the panel.
       height = Math.max(height, this.viewHeight);
       this.viewNode.style.maxHeight = `${height}px`;
     } else {
       this.browser.style.width = `${width}px`;
+      this.browser.style.minWidth = `${width}px`;
       this.browser.style.height = `${height}px`;
+      this.browser.style.minHeight = `${height}px`;
     }
 
     let event = new this.window.CustomEvent("WebExtPopupResized", {detail});
     this.browser.dispatchEvent(event);
   }
 
   setBackground(background) {
     let panelBackground = "";