Firefox won't install add-ons even after clean install

Question

Add-on installation fails in one of the following ways:
1. Unexpected installation error -203, or
2. The add-on appears in the "Extensions" tab as "Not compatible with Firefox 3.6.3", even though it is.
Sometimes I get the first error, sometimes the second, but all attempts to install add-ons fail with one of them.
I have tried all the troubleshooting advice in the knowledge bases (e.g. http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_-_Firefox#Unexpected_installation_error_-203) but with no success. I even did a clean reinstall of Firefox in a new directory (as described here: http://kb.mozillazine.org/Uninstalling_Firefox#On_Mac_OS_X) but the problem remains.
What else can I do?
== This happened ==
Every time Firefox opened
== I tried to install a new add-on. ==
== User Agent ==
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_3; en-US) AppleWebKit/533.4 (KHTML, like Gecko) Chrome/5.0.375.38 Safari/533.4

Answer 1

I have the same problem : impossible to install or update extensions; either by download (error -228) or by installing a local file (error -203). This is true for all situations : existing or new Firefox profile, different system users, etc.
I searched the kb, asked Google and so, but no solutions worked. The problem happens on all current versions of Firefox : stable, nightly,32 bits and 64 bits.
File extensions.log contains lines of
<code>
2010-04-02 21:36:38 - ExtensionManager:_finishOperations - failure, catching exception - lineno: 359 - file: undefined - [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIProperties.get]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: file:///Applications/Web/Firefox.app/Contents/MacOS/components/nsExtensionManager.js :: getDirInternal :: line 359" data: no]
</code>
My system is :
<code>
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; fr-FR; rv:1.9.3) Gecko/20100412 Minefield/3.7
Application Basics
Name
Firefox
Version
3.7
Profile Directory
Show in Finder
Installed Plugins
about:plugins
Build Configuration
about:buildconfig
Extensions
Name
Version
Enabled
ID
Adblock Plus
1.1.3
true
Adblock Plus: Element Hiding Helper
1.0.6
true
[email protected]
Add-on Collector
1.1.1
false
[email protected]
BetterPrivacy
1.47
true
Brief
1.2.5
true
[email protected]
British English Dictionary
1.19
false
[email protected]
BugMeNot
2.2
true
{987311C6-B504-4aa2-90BF-60CC49808D42}
CustomizeGoogle
0.76
true
Dictionary Switcher
1.0.1
true
[email protected]
Dictionnaire français «Réforme 1990»
3.5
true
[email protected]
Diigo Bookmarks and Web Annotations
4.1.0.46
true
Firefox PDF Plugin for Mac OS X
1.1.3
true
[email protected]
FoxyProxy Standard
2.19.1
false
[email protected]
Get jetable mail
0.4.1
true
{26af1522-982e-c0c4-f54a-7e69fb6432f5}
It's All Text!
1.4.1
true
[email protected]
Lazarus: Form Recovery
2.0.5
true
[email protected]
Personas
1.5.2
true
[email protected]
Tab in Textarea
0.9
true
Torbutton
1.2.4
false
URL Tooltip
1.2.1
true
[email protected]
User Agent Switcher
0.7.2
true
Xmarks
3.5.7
true
[email protected]
Zotero
2.0.2
false
[email protected]
Image Zoom
0.4.4
true
{1A2D0EC4-75F5-4c91-89C4-3656F6E44B68}
Firebug
1.5.3
false
[email protected]
Modified Preferences
Name
Value
accessibility.typeaheadfind.flashBar
0
browser.history_expire_days.mirror
180
browser.history_expire_days_min
7
browser.places.importBookmarksHTML
false
browser.places.smartBookmarksVersion
2
browser.startup.homepage
about:mozilla
browser.startup.homepage_override.mstone
rv:1.9.3
browser.tabs.warnOnClose
false
dom.disable_window_move_resize
true
dom.disable_window_status_change
false
dom.event.contextmenu.enabled
false
extensions.checkCompatibility
false
extensions.checkCompatibility.3.6
false
extensions.checkCompatibility.3.6b
false
extensions.checkCompatibility.3.7
false
extensions.checkCompatibility.3.7a
false
extensions.checkCompatibility.3.7a5
false
extensions.checkCompatibility.3.7b
false
extensions.lastAppVersion
3.7
font.minimum-size.x-western
11
font.name.monospace.x-western
DejaVu Sans Mono
font.name.sans-serif.x-western
DejaVu Sans
font.name.serif.x-western
DejaVu Serif
general.useragent.extra.betterprivacy
general.useragent.locale
fr-FR
javascript.options.showInConsole
true
keyword.URL
https://[www.google.com]/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=
network.cookie.prefsMigrated
true
network.http.max-connections
60
network.http.max-connections-per-server
30
network.http.max-persistent-connections-per-server
8
network.http.pipelining
true
network.http.pipelining.maxrequests
8
network.http.pipelining.ssl
true
network.http.proxy.pipelining
true
network.security.ports.banned
8118,8123,9050,9051
places.history.expiration.transient_current_max_pages
96636
places.last_vacuum
1276761638
print.macosx.pagesetup-2
PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTgiPz4KPCFET0NUWVBFIHBsaXN0IFBVQkxJQyAiLS8vQXBwbGUvL0RURCBQTElTVCAxLjAvL0VO…
print.print_bgcolor
true
print.print_bgimages
true
print.print_command
print.print_downloadfonts
false
print.print_evenpages
true
print.print_footerleft
print.print_footerright
&PT
print.print_headerleft
print.print_headerright
&D
print.print_in_color
true
print.print_margin_bottom
0.5
print.print_margin_left
0.5
print.print_margin_right
0.5
print.print_margin_top
0.5
print.print_oddpages
true
print.print_orientation
0
print.print_pagedelay
500
print.print_paper_data
0
print.print_paper_height
11.00
print.print_paper_size_type
1
print.print_paper_size_unit
0
print.print_paper_width
8.50
print.print_printer
print.print_reversed
false
print.print_scaling
1.00
print.print_shrink_to_fit
true
print.print_to_file
false
print.print_unwriteable_margin_bottom
57
print.print_unwriteable_margin_left
25
print.print_unwriteable_margin_right
25
print.print_unwriteable_margin_top
25
privacy.cpd.cookies
false
privacy.cpd.formdata
false
privacy.cpd.sessions
false
privacy.sanitize.migrateFx3Prefs
true
privacy.sanitize.timeSpan
0
security.crl.autoupdate.dayCnt.
1
security.crl.autoupdate.enable.
true
security.crl.autoupdate.errCount.
0
security.crl.autoupdate.errDetail.
Download of the CRL failed due to Network problems.
security.crl.autoupdate.freqCnt.
1
security.crl.autoupdate.nextInstant.
03/07/10 23:00:00
security.crl.autoupdate.timingType.
1
security.crl.autoupdate.url.
http://crl.certificat.com/DGI/DGI_UsagerB.crl
security.disable_button.openCertManager
false
security.warn_viewing_mixed
false
</code>
I am making heavy use of soft links on my systems : ~/Library/* (temp, cache, preferences, etc) is spread onto several physical volumes, and the filetree is done with soft links. No applications has any problem, except Firefox.