I've been having terrible trouble with Firefox crashing. Today I looked at about:crashes and found out symhtml.dll is one of the files causing Firefox to crash. I've also read EXCEPTION_ACCESS_VIOLATION_READ when I look at the submitted crash report.
Also;
couictlr.dll belonging to Norton Confidential
cowpplg.dll belonging to Norton Confidential
coffpl34.dll belonging to Norton Identity Safe
Originally I thought it was caused by Adobe Flash Player because C:\Users\<accountname>\AppData\Local\CrashDumps folder mentioned plugin-container.exe and Firefox has less crashes with Adobe Flash 15.0.0.189 but now I believe it's Norton Toolbar.