Do you have Add-on Toolbar checked? That selects it to display; un-checked = don't display. Click to check or un-check. See https://support.mozilla.com/en-US/kb/Back%20and%20forward%20or%20other%20toolbar%20items%20are%20missing

• View > Toolbars
• Firefox button > Options > Toolbar Layout
• Right-click in empty space at the end of the Tab Bar

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Not related to your question.

You have multiple Java Console extensions that Java did not clean up when updating. The old versions are: 6.0.19, 6.0.20, 6.0.21, 6.0.22, 6.0.23
You can remove them by following these instructions: http://kb.mozillazine.org/Java#Multiple_Java_Console_extensions

Although I appreciate your suggested resolution, the Add-on bar, in tool bars, is in fact already checked, which is one of the reasons I do not fully comprehend the reason for the Add-on tool bar disappearing, whenever exiting the customization screen.

The Add-on tool bar appears upon entering view>tool bars> customization and I am able to place other add-on options in the tool bar, but as soon as I click on "Done" or simply exit the customization function by closing the screen by clicking on the "x" in the upper right corner, the tool bar continually disappears.

I apologize for my apparent lack of knowledge and/or ability to locate the "Firefox button" from which I can go to options and I am precluded from utilizing Ctrl+/ in the space alluded to in the other possible option. (I will return to the "HELP" screen and "FAQs", in an effort to identify the aforementioned "Firefox button")

But, once again, I sincerely appreciate your willingness to provide me with a suggested resolution. Respectfully, 10pinfvr

P.S. Although unrelated, in reviewing the "Uninstall" feature or "ALL" programs feature, only version 26 appears when utilizing either methodology, so I am unable to uninstall Java versions 19 - 23, referred to in the message I received, since they are not visible to me in the only modalities of which I'm aware.

But "Thank-you" for the recommendation

Mea Culpa, I forgot to check the Firefox extensions. I have since removed Java versions 19 - 23.

Try:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Cor-el,

 Despite numerous attempts to utilize your suggested "fix" to the disappearing tool bar problem, the results remained unchanged (i.e. tool bar immediately appears for customization, but immediately disappears upon exiting the tool bar customization screen).
 In an effort to determine if transactional sequencing may have any impact on affecting the appearance/disappearance of the add-on tool bar, in addition to attempting to resolve the current challenge through the use of the Firefox safe mode and reset tool bar functionality, I also elected to change the sequence in which I modified the desired "customization(s)", as well as the methodology for exiting the customization screen, Firefox safe mode (e.g. click-on Done, exit through X, exit and renter FF safe mode, with and without closing the customization screen, etc...).
 Further in addition to following the documented transactions, I also exited and reentered the Windows 7 O.S., as well as exiting and reentering Firefox safe mode and Firefox. 
 Unfortunately, despite the myriad of methodologies I attempted, the resultant effect remained the same (i.e. immediate disappearance of add-on tool bar, when clicking-on Done or exiting the screen and/or system).
 However, I did notice one anomaly, which heretofore had gone unnoticed due to benefits garnered rather than features lost. 
 Since the beginning of the this recent disappearing tool bar challenge, the add-ons which I select and locate on the URL address line continue to remain on screen, despite the disappearance of the add-on tool bar and all selected add-on "customizations" reflected in the add-on tool bar, regardless of any/other changes.   
 I want to thank you for your valuable contribution to the "hopeful" resolution of this most recent problem and I sincerely apologize for the delay in responding your suggestion. (During the past few days, we have been entertaining a relative from another country and as a result, I elected not to spend time on the computer).  Respectfully, 10pinfvr

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes