Strikethrough didn't show correctly when font size is changed

[Kinny]

Strikethrough didn't show correctly when font size is changed. And this problem only happens in Firefox.

Steps to reproduce

1. From CKEditor demo page ​http://ckeditor.com/demo#full, enter some Normal text (Format is Normal and size is 12)
2. Select the text typed in step 1 and click Strikethrough
3. Place the cursor in the beginning of the text from step 1
4. Change font size to 48 and type other text

The strikethrough line appears according to the previous text size. It showed at the bottom of the new text instead of in the middle.

Expected result

The strikethrough line appears in the middle of the text.

Actual result

The strikethrough line appears according to the previous text size. It showed at the bottom of the new text instead of in the middle.

Other details (browser, OS, CKEditor version, installed plugins)

This problem only happen in FireFox.

[Jakub Ś]

DUP of #8118.