Well if a different browser is what will solve your problems, than maybe you should take a look at some of them.

The one thing I am not seeing you talk about is Profile Manager. You did mention creating new profiles but nothing about using Profile Manager to run them. You can not run the same profile for these two versions. It will corrupt Firefox to the ninth degree. You must invoke the Program Manager for each version you run or add the profile to your start command line.

Thank you for your reply, DPMR.

The only mention I made of profiles was that it was a potential next step (and that, unfortunately, was in the second box of the ask question form, and is only visible from the "More Sytem Details" link, above. That's also where I noted the steps I took, but I didn't realize it wouldn't be visible as part of the question. I didn't do anything specifically in regard to profiles other than whatever changes the installation programs made.

Anyhow, I did find a solution eventually.

With further reading, I found a link somewhere to a page about the file, "localstore.rdf". From the knowledgbase section :

http://kb.mozillazine.org/Localstore.rdf

Despite its warning that it's "not advisable to edit", it wasn't too terribly hard to find the pertinent entry:

 <RDF:Description 

RDF:about="chrome://browser/content/browser.xul#TabsToolbar"

                  currentset="new-tab-button,tabbrowser-tabs,noscript-tbb-revoke-temp,noscript-tbb-temp-page,noscript-tbb,alltabs-button,tabs-closebutton,window-controls" />

Before I did anything, I made a copy of the file and put it in a safe folder elsewhere. I won't go into the details of the various edits I tried, putting the 'new-tab-button' entry into different positions, none of which achieved what I needed.

One option was to delete the localstore.rdf file (see knowledgebase), but that results in a default setup, losing all the changes that have been made. I finally tried just deleting the entry above from the file, and that restored my 'new tabs +' button.

If other changes have been made to the 'Tabs-To0lbar', this may return those to default. The only entry I have now in my localstore.rdf relating to this is:

RDF:resource="chrome://browser/content/browser.xul#TabsToolbar"/>

 </RDF:Description>

which I suspect may be the default configuration, but I don't know that for sure.

There was a question posted about the same problem somewhere, I've lost the source now, that noted, oddly, that even though the +box is not visible, double-clicking on the spot next to the tab that's on the rightmost end of the line of tabs, where the +box should be, works to open a new tab; as though the button is there but has just become invisible. I tried this and it does work for me, but I'm happier that I have the +box back again.

Another possible solution was posted by REWalker, which I'll copy here in case someone else with this problem finds this post, but not that one:

"Here is what I did to resolve this.

"Open the "Customize" window (right click to the right of any tab and select "Customize", then drag the "New Tab" button that is to the left of the URL window off the page, and back into the Customize Screen.

"Then drag that icon back onto the tab bar; just to the right of any open tab. If your browser is like mine, that icon will appear as the old "+" but will be on the LEFT of your open tabs. Close the Customize screen, then re-open Customize and drag that "+" icon to the right of your open tabs, and it should stay there.

"That worked for me; hopefully it'll help some of you... "

REWalker

That actually did not work for me. The "+" would never stay attached to the right of the open tabs, always insisting on going back to the far left.

Anyhow, thank you for the reply.

Good you found a solution. You should mark this solved so others can see you found a solution.