Which email provider are you using? (It must be something specific to their website, as Firefox itself doesn't send emails, with or without attachments).
If you want to get Firefox back to its 'out of the box' state though, you don't need to reinstall it:
Click the 'hamburger' (three horizontal lines, top right) and then 'Add-ons'. If 'Extensions' isn't already selected (on the left), click on it. Note down the names of any extensions you're using (such as Adblock Plus) which you want to keep. Close the tab.
If you've not got the Menu Bar (File, Edit, View, etc) across the top of your screen, enable it by right-clicking at the top of the page and ticking the relevant box. Then go to Help > Troubleshooting Information. Click on 'Refresh Firefox'. Then reinstall the extensions you wanted to retain.