Can't install a search plugin

Question

OS X 10.8.5, Firefox 24.5.0
1. Click on dropdown in Firefox's search field.
2. A list of installed search engine plugins appears along with "Manage Search engines...". Click this.
3. Click on "Get more search engines"
4. https://addons.mozilla.org/en-US/firefox/search/?atype=4 opens in a new tab. Select DuckDuckGo HTTPS/SSL or YouTube.
5. Click the green "+ Add to Firefox" button.
6. Firefox/mozilla.org complains in a dialog box: "Sorry, you need a Mozilla-based browser (such as Firefox) to install a search plugin."
Oops, I'm using Firefox 24.5.0.
Any ideas how to fix this? Thanks.
Info from Help>Troubleshooting is below.
==============================
Application Basics
Name: Firefox
Version: 24.5.0
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Firefox/24.0
Extensions
Name: Adblock Plus
Version: 2.0.3
Enabled: true
ID: {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}
Name: BetterPrivacy
Version: 1.68
Enabled: true
ID: {d40f5e7b-d2cf-4856-b441-cc613eeffbe3}
Name: Clip to DEVONthink
Version: 1.2.3
Enabled: true
ID: [email protected]
Name: Collusion
Version: 0.16.3
Enabled: true
ID: jid1-F9UJ2thwoAm5gQ@jetpack
Name: DoNotTrackPlus
Version: 2.0.9
Enabled: true
ID: [email protected]
Name: DownloadHelper
Version: 4.9.15
Enabled: true
ID: {b9db16a4-6edc-47ec-a1f4-b86292ed211d}
Name: DownThemAll!
Version: 2.0.13
Enabled: true
ID: {DDC359D1-844A-42a7-9AA1-88A850A938A8}
Name: Element Hiding Helper for Adblock Plus
Version: 1.2.1
Enabled: true
ID: [email protected]
Name: Flashblock
Version: 1.5.17
Enabled: true
ID: {3d7eb24f-2740-49df-8937-200b1cc08f8a}
Name: Google/Yandex search link fix
Version: 1.4.1
Enabled: true
ID: jid0-XWJxt5VvCXkKzQK99PhZqAn7Xbg@jetpack
Name: Lucifox
Version: 0.8.4
Enabled: true
ID: [email protected]
Name: Remove Cookies for Site
Version: 0.62
Enabled: true
ID: {06997db0-c027-4d5f-bd37-b0d9230226ea}
Name: ScrapBook
Version: 1.5.3
Enabled: true
ID: {53A03D43-5363-4669-8190-99061B2DEBA5}
Name: User Agent Switcher
Version: 0.7.3
Enabled: true
ID: {e968fc70-8f95-4ab9-9e79-304de2a71ee1}
Name: Disconnect
Version: 1.0.1
Enabled: false
ID: [email protected]
Name: selectivecookiedelete
Version: 4.1
Enabled: false
ID: [email protected]
Important Modified Preferences
accessibility.typeaheadfind.flashBar: 0
browser.cache.disk.capacity: 307200
browser.cache.disk.smart_size_cached_value: 768000
browser.cache.disk.smart_size.enabled: false
browser.cache.disk.smart_size.first_run: false
browser.cache.disk.smart_size.use_old_max: false
browser.display.use_document_fonts: 0
browser.history_expire_days.mirror: 180
browser.places.smartBookmarksVersion: 4
browser.sessionstore.restore_on_demand: false
browser.startup.homepage: file:///Volumes/Data/Oro/_Blank Page.html
browser.startup.homepage_override.buildID: 20140421162246
browser.startup.homepage_override.mstone: 24.5.0
dom.mozApps.used: true
extensions.lastAppVersion: 24.5.0
font.minimum-size.x-western: 13
font.name.monospace.x-western: Monaco
font.name.sans-serif.x-western: Lucida Sans
font.name.serif.x-western: Calibri
font.size.variable.x-western: 13
gfx.blacklist.webgl.msaa: 4
network.cookie.prefsMigrated: true
places.database.lastMaintenance: 1399518177
places.history.expiration.transient_current_max_pages: 104858
places.last_vacuum: 1331401559
plugin.disable_full_page_plugin_for_types: application/pdf
plugin.importedState: true
plugin.state.iphotophotocast: 0
plugin.state.java: 0
plugin.state.sharepointbrowserplugin: 0
print.macosx.pagesetup-2: PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTgiPz4KPCFET0NUWVBFIHBsaXN0IFBVQkxJQyAiLS8vQXBwbGUvL0RURCBQTElTVCAxLjAvL0VO
print.print_bgcolor: false
print.print_bgimages: false
print.print_colorspace:
print.print_command:
print.print_downloadfonts: false
print.print_duplex: 0
print.print_evenpages: true
print.print_footerleft: &U
print.print_footerright:
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_page_delay: 50
print.print_pagedelay: 500
print.print_paper_data: 0
print.print_paper_height: 11.00
print.print_paper_name:
print.print_paper_size_type: 1
print.print_paper_size_unit: 0
print.print_paper_width: 8.50
print.print_plex_name:
print.print_printer:
print.print_resolution: 0
print.print_resolution_name:
print.print_reversed: false
print.print_scaling: 0.60
print.print_shrink_to_fit: false
print.print_to_file: false
print.print_unwriteable_margin_bottom: 17
print.print_unwriteable_margin_left: 25
print.print_unwriteable_margin_right: 25
print.print_unwriteable_margin_top: 17
privacy.clearOnShutdown.cookies: false
privacy.clearOnShutdown.downloads: false
privacy.clearOnShutdown.formdata: false
privacy.clearOnShutdown.history: false
privacy.clearOnShutdown.sessions: false
privacy.cpd.cookies: false
privacy.cpd.downloads: false
privacy.cpd.formdata: false
privacy.cpd.history: false
privacy.cpd.sessions: false
privacy.donottrackheader.enabled: true
privacy.sanitize.migrateFx3Prefs: true
privacy.sanitize.sanitizeOnShutdown: true
privacy.sanitize.timeSpan: 0
security.OCSP.require: true
security.warn_viewing_mixed: false
storage.vacuum.last.index: 1
storage.vacuum.last.places.sqlite: 1399078504
Graphics
Device ID: 0x 166
GPU Accelerated Windows: 2/2 OpenGL
Vendor ID: 0x8086
WebGL Renderer: Intel Inc. -- Intel HD Graphics 4000 OpenGL Engine
windowLayerManagerRemote: false
AzureCanvasBackend: quartz
AzureContentBackend: none
AzureFallbackCanvasBackend: none
JavaScript
Incremental GC: true
Accessibility
Activated: false
Prevent Accessibility: 0
Library Versions
NSPR
Expected minimum version: 4.10.2
Version in use: 4.10.2
NSS
Expected minimum version: 3.15.4 Basic ECC
Version in use: 3.15.4 Basic ECC
NSSSMIME
Expected minimum version: 3.15.4 Basic ECC
Version in use: 3.15.4 Basic ECC
NSSSSL
Expected minimum version: 3.15.4 Basic ECC
Version in use: 3.15.4 Basic ECC
NSSUTIL
Expected minimum version: 3.15.4
Version in use: 3.15.4

Answer 1

Try to install that search engine in Safe Mode if it doesn't work in normal mode.
Start Firefox in <b>[[Safe Mode | Safe Mode]]</b> to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
*Do not click the Reset button on the Safe mode start window or otherwise make changes.
*'''Windows/Mac''': Go to Help > Restart with add-ons disabled
*'''Linux''': Run ''firefox -safe-mode'' in the Terminal/Konsole