Arrow buttons for shifting through tabs stop working

Question

On my Linux Ubuntu system with Firefox 17.0.1 (and with several earlier releases, eg 12-16 I think) after a few weeks the tab triangle or arrowhead on one or more Firefox windows becomes defective. This problem is different from http://support.mozilla.org/en-US/questions/931515 in that if I restart the browser and resume the Firefox session the problem goes away for a while. (Note: I will wait until 05:00 UTC on 28 Dec before restarting FF to clear the current instance of the problem, in case anyone can suggest other relevant tests or data.)
The main symptoms are these: Triple click no longer slides the list of tabs all the way left or right. Double click appears to work once (from an extreme end). Single clicks step the tab list one tab at a time up to near an extreme end but then stop working before the extreme tab becomes visible. The following paragraphs give some more details about the symptoms.
When the problem occurs, tabs for new pages (eg reached by links from the right-most visible tab and opened in new tabs) do not become visible (that is, are not squeezed into the list on the tabs bar). The right-facing triangle, although not grayed out, does not make the tabs for the newly opened pages visible, but seems to have no effect. Clicking the X for the right-most visible tab closes that page and then one of the tabs that are "hidden out to the right" becomes visible, but other tabs out to the right remain inaccessible.
Meanwhile, the tab triangle arrowheads continue to work ok on other FF windows. For example, the left and right tab triangles in this current FF window (where 15 tabs are visible for 17 pages open in the window) are working fine. But in the FF window with the problem (where 15 tabs are visible for 40 pages open in the window) the triangles stop working one tab too soon: they don't give access to the leftmost or rightmost tab.
Note, clicking + (Open a new tab) jumped the list to the right end, with the new tab and an empty page visible; but after I clicked the left triangle three times, so that the new tab scrolled off to the right, no matter how many times I click the right triangle, that tab remains invisible, even though the webpage for it is still visible as the currently selected page. Opening a URL from command line acts the same way.
The system runs Linux 3.2.0-35-generic #55-Ubuntu 12.04.1 LTS x86_64 system with Firefox 17.0.1. I don't remember when I first encountered the problem but believe it was in 2011 (vs 2010) on a Fedora 7 system. Restarting FF has always cleared the problem. The command line "echo session $DESKTOP_SESSION desktop $XDG_CURRENT_DESKTOP; wmctrl -m" produces (in part)
session ubuntu-2d desktop Unity
Name: Metacity
"top" shows, in part:
PID PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3033 20 0 2158m 1.1g 38m S 7 28.7 3878:21 firefox
Note, I snipped 1200 lines of text from the Troubleshooting Information because the 50000 bytes in it pushed this post's length over the 30000 byte limit. All of the snipped lines started with "print.tmp.printerfeatures"

Answer 1

Firefox left/right arrows are broken.
When you have multiple tabs open and you use the left and right arrows to scroll through your open tabs, they stop at a certain point, you then have to click the little, "List all tabs" icon in order to access one of your open tabs.
This is on a Windows 7 64-Bit OS. I've already followed all the instructions, still no good. This is Firefox version 19.0.2