Underlined texts in a page from an imported Word doc results in a strange behavior in Firefox

This bug only occurs in Firefox. Underlined texts in a page from an imported Word doc results in a strange behavior in Firefox.

Steps to reproduce

1. Import the Word document test.doc into Confluence
2. Edit the page
3. Select/highlight the NOTE: text and then delete it

Result: The whole sentence is underlined as a result of the deletion.

Reason
When a Word document is imported into Confluence, any underlined texts are converted into the following HTML:

<span style="text-decoration: underline;">underlined text</span>

It should be converted using <u> tag for underline, like this:

<u>underlined text</u>

Workaround
Highlight/select the underlined text, click the "Underline" style button in the toolbar (Ctrl+U) to clear the underline. Click the button again to make the underline appears. Deleting the text will not result in an above scenario.