AFAIK, SUMO doesn't use Flash for anything.

Can I have your troubleshooting information?

• Go to Help > Troubleshooting Information > Copy text to clipboard and paste (CTRL + V/Command + V) your response into a new reply.

Thank you. Here you go!

Application Basics
------------------
Name: Firefox
Version: 28.0
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:28.0) Gecko/20100101 Firefox/28.0
Crash Reports for the Last 3 Days
---------------------------------
All Crash Reports
Extensions
----------
Name: avast! Online Security
Version: 8.0.1506
Enabled: true
ID: [email protected]
Name: Flash and Video Download
Version: 1.53
Enabled: true
ID: {bee6eb20-01e0-ebd1-da83-080329fb9a3a}
Name: Flash Video Downloader - Full HD Download
Version: 5.8.1
Enabled: true
ID: [email protected]
Name: Javadoc Lookup Extension
Version: 1.0.1
Enabled: true
ID: [email protected]
Name: JavaScript Debugger
Version: 0.9.89
Enabled: true
ID: {f13b157f-b174-47e7-a34d-4815ddfdfeb8}
Name: JavaScript Deobfuscator
Version: 1.6.4
Enabled: true
ID: [email protected]
Name: QuickJava
Version: 2.0.4
Enabled: true
ID: {E6C1199F-E687-42da-8C24-E7770CC3AE66}
Important Modified Preferences
------------------------------
accessibility.typeaheadfind.flashBar: 0
browser.cache.disk.capacity: 358400
browser.cache.disk.smart_size_cached_value: 358400
browser.cache.disk.smart_size.first_run: false
browser.cache.disk.smart_size.use_old_max: false
browser.places.smartBookmarksVersion: 6
browser.sessionstore.upgradeBackup.latestBuildID: 20140314220517
browser.startup.homepage_override.buildID: 20140314220517
browser.startup.homepage_override.mstone: 28.0
dom.disable_open_during_load: false
dom.max_script_run_time: 0
dom.mozApps.used: true
dom.w3c_touch_events.expose: false
extensions.lastAppVersion: 28.0
font.internaluseonly.changed: true
javascript.enabled: false
network.cookie.prefsMigrated: true
places.database.lastMaintenance: 1398135861
places.history.enabled: false
places.history.expiration.transient_current_max_pages: 104858
plugin.disable_full_page_plugin_for_types: application/pdf
plugin.importedState: true
plugin.state.adobepdfviewernpapi: 2
plugin.state.java: 2
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_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_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_resolution: 0
print.print_resolution_name:
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: 56
print.print_unwriteable_margin_left: 25
print.print_unwriteable_margin_right: 25
print.print_unwriteable_margin_top: 25
privacy.clearOnShutdown.cache: false
privacy.clearOnShutdown.cookies: false
privacy.clearOnShutdown.downloads: false
privacy.clearOnShutdown.formdata: false
privacy.clearOnShutdown.history: false
privacy.clearOnShutdown.sessions: false
privacy.sanitize.migrateFx3Prefs: true
privacy.sanitize.sanitizeOnShutdown: true
privacy.sanitize.timeSpan: 0
security.warn_viewing_mixed: false
storage.vacuum.last.index: 1
storage.vacuum.last.places.sqlite: 1396917484
Graphics
--------
Device ID: 0x 166
GPU Accelerated Windows: 1/1 OpenGL (OMTC)
Vendor ID: 0x8086
WebGL Renderer: Intel Inc. -- Intel HD Graphics 4000 OpenGL Engine
windowLayerManagerRemote: true
AzureCanvasBackend: quartz
AzureContentBackend: quartz
AzureFallbackCanvasBackend: none
AzureSkiaAccelerated: 0
JavaScript
----------
Incremental GC: true
Accessibility
-------------
Activated: false
Prevent Accessibility: 0
Library Versions
----------------
NSPR
Expected minimum version: 4.10.3
Version in use: 4.10.3
NSS
Expected minimum version: 3.15.5 Basic ECC
Version in use: 3.15.5 Basic ECC
NSSSMIME
Expected minimum version: 3.15.5 Basic ECC
Version in use: 3.15.5 Basic ECC
NSSSSL
Expected minimum version: 3.15.5 Basic ECC
Version in use: 3.15.5 Basic ECC
NSSUTIL
Expected minimum version: 3.15.5
Version in use: 3.15.5

Can you try to start Firefox in Safe Mode to see if Firefox works properly with no error? You can start Firefox in Safe Mode below:

• Windows/Mac: Go to Help > Restart with add-ons disabled
• Linux: Run firefox -safe-mode in the Terminal/Konsole

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

Make sure Flash is up to date. Adobe released a patch to the latest version of Flash that made Flash crash in older model Macs so be sure to download it below if you haven't:

• https://get.adobe.com/flashplayer

Try to see if Flash is actually enabled by checking to see if the website below displays your Flash Player version, OS, and browser. If it doesn't it shows a blank box. What do you see?

• http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-mac.html

I ended up removing the firefox profile folder from the library, uninstalled, re-installed, and flash seems to be back!!  :-D