1. Copy URL to clipboard
2. Highlight the text to turn into a link
3. Click the link icon
4. Click "web address"
   
5. Paste the URL in the "Link to this URL"
6. Click the OK button

Now that process is no longer working for Firefox 9 users. The text highlighted in step 2 doesn't appear in the "Text to display" field (it is instead blank) and whatever is pasted into the "Link to this URL" field simultaneously appears in the "Text to display" field.

Google are aware and are working on a fix