Every email site has an icon to add an attachment to a message. Often there's an icon similar to a paper clip. That should pop up a dialog to browse your computer for the file so you can attach it.

Also, your Firefox identified itself to the forum as version 22. Is that correct? Version 22 is not secure; Mozilla discloses [https://www.mozilla.org/security/known-vulnerabilities/firefox.html security flaws] after each new release. Is something holding you back from upgrading to Firefox 47? Please let us know so we can suggest solutions or workarounds. Or you can update now using the steps in this article: [[Update Firefox to the latest version]]. If Help > About Firefox shows Firefox 47.0, you may need to clear the preference that is causing Firefox to misreport your version number. See: [[Websites say that Firefox is outdated or incompatible even though it's the latest version]].