Enable first line title parsing for Firefox

[mkaz]

This PR removes the check for Firefox that disables first line title parsing.
The reasoning of not supporting ::first-line is no longer valid.
https://caniuse.com/#feat=css-first-line

Test:

1. Apply PR
2. Add new post with a suitable first line title
3. Confirm title is added as an H1 to the post, and set in wp-admin

[mkaz]

Correction: The first-line selector does not work within a Textarea in Firefox. So the formatting can not be displayed. The parsing of the title in the first line does work, but is not shown visually from within the textarea on Firefox.