Снес All-In-One_Sidebar и всё прошло.

Расширения устанавливаются, пока не установлена галочка в настройках Local Install'а "Автоматически сохранять установленные расширения и темы" или пока в полях "Пути установки" стоят значения "H:"

#  Added: Massive updates to support new Addons interface for Extensions and Themes, should be 99.9999% compatible with Bon Echo 2.0 and Minefield 3.0
# Added: Hidden hack to hide Menu icons has now been super-streamlined and added as a preference in the Menu Options|Miscellaneous tab of the Options window (css hack no longer needed and reusing hidden pref of "local_install.hideMenuIcons")
# Added: You can now add "Tab Count" and "Window Count" to the customized window title, for example (${TabCount}/${WindowCount}) would yield (5/2) if five tabs were open in the current window and there were two browser windows open, check General|Window Title Options in the Local Install options window
# Added: You can now move the Extension/Themes/Addons button bar above the list view, check Manager Hacks|Layout in the Local Install options window (currently not compatible with Bon Echo 2.0 but works with 1.0.x, 1.5 & 3.0 builds)
# Added: "Show Extension Directory" is now on the list of extensions to prompt to uninstall when you open the Extension manager window as this functionality is included with Local Install.
# Added: For Sunbird users "about:config" is now a toolbar icon
# Added: Hidden setting to Show Attachment box for Email Compose window now has a Option window setting in the Miscellaneous tab
# Added: Hidden setting to Always save Chrome Editor files now has a Option window setting in the My Config|Chrome Editor tab
# Updated: Properly setting options tabs when no archive directory is set and user selects Archive from context menu
# Updated: Now hiding Nightly Tester Tool button image if hide default images settings is enabled
# Updated: about:plugins and about:config toolbar context menus now include recently added about: functions
# Updated: removed some unused Sunbird context menus till I get a chance to fix them, saving about 2k
# Updated: fr-FR locale updated
# Updated: Moved "Hide 'Paste userAgent/Build ID' from Textbox/Input Context Menu" to the Menu Options|Miscellaneous tab of the Options window
# Fixed: Long standing issue with duplicate tabs being created for the Extension & Theme manager when installing and already having a tab open and additionally updated to include any time Extension/Theme is viewed in a tab it will refocus there despite target preference
# Fixed: Slight issue with supporting custom locale useragent in Disable XPI Delay install dialog
# Fixed: Theme switch was broken with older Firefox/Thunderbird 1.0.x & Netscape 8.x versions
# Fixed: Opening My Config to sidebar from the Extension/Theme Manager menu icon was not working if EM/TM was loaded in a window.
# Fixed: Text flicker on Extension install dialog if Nightly Tester Tools was installed also
# Fixed: bogus "gExtensionsView.selected" error message in Extension/Themes manager with context menus
# Fixed: properly hiding "Make Compatible" if no extension/theme is selected
# Fixed: disabling Extension Options and Theme Switcher for Mozilla Suite/Seamonkey in preferences as these options are not available with those apps
# Fixed: Preference listener leak in My Config window (found using Leak Monitor,http://dbaron.org/mozilla/leak-monitor/)