Copy and paste works for me.

Yes but that entails opening the file in another program then copy before pasting - a bit long winded when in OE it was one simple click. If you have text in more than one file to insert copy and paste then it's even more intensive. I use it for a variety of standard replies.

I suggest you investigate the use of templates. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Using_Templates

You might also look at some of the template add-ons that automate some of the actions https://addons.mozilla.org/en-US/thunderbird/search/?q=templates

Quicktext works perfectly. Goes beyond OE 'insert text from file'.