Firefox has had rounded tabs for over a year now, since the release of Firefox 29. Were you using an add-on to change the appearance?

In case it was disabled because it needs to be updated, you might try updating extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, above the list, click the "gear" button and Check for Updates.

If your extensions are GONE, then that's the problem. Did Firefox's Refresh feature run recently? The tell-tale sign is an Old Firefox Data folder on your desktop that contains a semi-randomly-named folder with freshly updated files inside.

I guess a more general response would be this article from around that time: How to make the new Firefox look like the old Firefox.