It has been removed due to changes to Firefox known under the codename Australis.

For more information and for workarounds please see

• my orange firefox button disappeared, and I do not have the menu toolbar open /questions/991086#answer-546555

As for the reminderfox bowtie icon that is not something I use but have you checked to see if it may now be added to your toolbar as a customisation ?
Try clicking on the new button, then the customise option you may get an option to add the bowtie icon to either the menu or a toolbar. If not try the addon's support site to see if they can assist.

You can install the Classic Theme Restorer extension to restore some functionality that was lost with the arrival of Australis.

• https://addons.mozilla.org/firefox/addon/classicthemerestorer/

• http://www.mozilla.org/en-US/firefox/29.0beta/releasenotes/
• http://www.ghacks.net/2014/01/31/mozilla-decides-uplift-australis-firefox-aurora/