Here are a few steps you can try to troubleshoot and resolve the problem: Clear Browser Cache and Cookies: Clear your browser's cache and cookies in Firefox. Sometimes, stored data can cause unexpected issues. After clearing, restart the browser and try attaching the file again. Try a Different Browser Version: Ensure that you are using the latest version of Firefox. If not, consider updating to the latest version as updates may include fixes for compatibility issues. Browser Add-ons/Extensions: Temporarily disable any browser add-ons/extensions, as they might interfere with the attachment process. If the issue is resolved, re-enable them one by one to identify the culprit. Safe Mode: Try using Firefox in Safe Mode. Safe Mode disables add-ons and uses default settings. If the problem disappears in Safe Mode, an add-on may be causing the issue. Check File Type and Size: Verify that the file type is supported by Outlook Web App. Additionally, check the file size. While you mentioned the file is only 26KB, ensure it meets the attachment size limits set by your email provider. Reset Firefox Settings: Resetting Firefox to its default settings can resolve various issues. Be cautious as this will remove your add-ons and customizations. You can find this option in Firefox's settings. Try Private Browsing Mode: Attempt to attach the file while in Firefox's private browsing mode. This mode runs the browser without saving history, cookies, or site data. Update JavaScript: Ensure that JavaScript is enabled and up to date in your Firefox settings, as OWA heavily relies on JavaScript. Since you mentioned it worked in Chrome, consider using Chrome temporarily for attaching files until the issue in Firefox is resolved. Sometimes it happens when I work on my website [http://example.com beach buggy racing keyboard controls] using Firefox.