From Crash reporter:

AdapterDeviceID: 0x3ea5 AdapterVendorID: 0x8086 BuildID: 20210503134251 ContentSandboxCapable: 1 ContentSandboxLevel: 3 CrashTime: 1621900976 DOMIPCEnabled: 1 Email: EventLoopNestingLevel: 1 FramePoisonBase: 9223372036600930304 FramePoisonSize: 4096 InstallTime: 1621899514 Notes: FP(D00-L1000-W00000000-T000) WR? WR- OMTP? OMTP- ProductID: {ec8030f7-c20a-464f-9b0e-13a3a9e97384} ProductName: Firefox ReleaseChannel: esr SafeMode: 1 SecondsSinceLastCrash: 8 StartupCrash: 0 StartupTime: 1621900974 TelemetryEnvironment: {"build":{"applicationId":"{ec8030f7-c20a-464f-9b0e-13a3a9e97384}","applicationName":"Firefox","architecture":"x86-64","buildId":"20210503134251","version":"78.10.1","vendor":"Mozilla","displayVersion":"78.10.1esr","platformVersion":"78.10.1","xpcomAbi":"x86_64-gcc3","updaterAvailable":true},"partner":{"distributionId":null,"distributionVersion":null,"partnerId":null,"distributor":null,"distributorChannel":null,"partnerNames":[]},"system":{"memoryMB":8192,"virtualMaxMB":null,"cpu":{"extensions":["hasMMX","hasSSE","hasSSE2","hasSSE3","hasSSSE3","hasSSE4_1","hasSSE4_2","hasAVX","hasAVX2","hasAES"]},"os":{"name":"Darwin","version":"18.7.0","locale":"en-GB"},"hdd":{"profile":{"model":null,"revision":null,"type":null},"binary":{"model":null,"revision":null,"type":null},"system":{"model":null,"revision":null,"type":null}},"gfx":{"D2DEnabled":null,"DWriteEnabled":null,"ContentBackend":"Skia","Headless":false,"adapters":[{"description":null,"vendorID":"0x8086","deviceID":"0x3ea5","subsysID":null,"RAM":0,"driver":null,"driverVendor":null,"driverVersion":null,"driverDate":null,"GPUActive":true}],"monitors":[{"screenWidth":1440,"screenHeight":900,"scale":2}],"features":{"compositor":"none","hwCompositing":{"status":"blocked:FEATURE_FAILURE_COMP_SAFEMODE"},"gpuProcess":{"status":"unused"},"wrQualified":{"status":"blacklisted:FEATURE_UNQUALIFIED_WEBRENDER_MAC"},"webrender":{"status":"unavailable-in-safe-mode"},"wrCompositor":{"status":"disabled"}}},"appleModelId":"MacBookPro15,2"},"settings":{"blocklistEnabled":true,"e10sEnabled":true,"e10sMultiProcesses":8,"telemetryEnabled":false,"locale":"en-US","intl":{},"update":{"channel":"esr","enabled":true},"userPrefs":{"browser.search.region":"DK","browser.search.widget.inNavBar":false,"browser.startup.homepage":"<user-set>","browser.startup.page":3,"fission.autostart":false,"privacy.donottrackheader.enabled":true},"sandbox":{"effectiveContentProcessLevel":3},"addonCompatibilityCheckEnabled":true,"isDefaultBrowser":null},"profile":{}} ThreadIdNameMapping: 6:"Gecko_IOThread",7:"Timer",10:"Socket Thread",11:"Permission",12:"JS Watchdog",14:"JS Helper",16:"JS Helper",15:"JS Helper",13:"JS Helper",18:"JS Helper",17:"JS Helper",19:"JS Helper",20:"JS Helper",22:"Cache2 I/O",23:"Cookie",24:"StreamTrans #3",25:"Worker Launcher",26:"Compositor",27:"ImgDecoder #1",28:"ImageIO",29:"IPDL Background",31:"DOM Worker",31:"QuotaManager IO",33:"IndexedDB #1",34:"StyleThread#0",35:"StyleThread#1",36:"StyleThread#2",37:"StyleThread#3",38:"StyleThread#4",39:"StyleThread#5",40:"StreamTrans #4",41:"StreamTrans #5",45:"ImageBridgeChld",46:"TRR Background",48:"DOM Worker",49:"StreamTrans #8",48:"StreamTrans #9", Throttleable: 1 UptimeTS: 1.98801487 Vendor: Mozilla Version: 78.10.1 useragent_locale: en-US

This report also contains technical information about the state of the application when it crashed.

Product Firefox Release Channel esr Version 78.10.1esr Build ID 20210503134251 (2021-05-03) Buildhub data OS OS X 10.14 OS Version 10.14.6 18G6020

bp-5d23281d-a7aa-4ab2-b68a-1faab0210524 Signature: _platform_strcmp | pref_Lookup

Uptime 3 seconds

Crash Reason EXC_BAD_ACCESS / EXC_I386_GPFLT

Make sure all software is up to date.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Hi, thanks for your response. As stated in the title, Firefox won't even start in Safe Mode and Crash Reporter automatically appears. I was running OS Mojave yesterday (10.14 I think) and updated overnight to Big Sur (11.4) but the problem persists. I've tried reinstalling Firefox but still can't open it.

I take that back. I just deleted Firefox and reinstalled it again on Big Sur and it's working fine.

That was very good work. Well Done.