From what I have been reading from other posts on this forum, the issue may be caused by the clear history settings.

Press the <Alt Key> to bring up the tool bar. Then Tools > Options > Privacy. The button next to History, select Use Custom Settings.

At the bottom of the page, turn on Clear History When Firefox Closes.
At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

https://support.mozilla.org/questions/997866?esab=a&s=&r=1&as=s
This is not a cure but will make it easier if Firefox locks up.

See also the Firefox 29 release notes:

• http://www.mozilla.org/en-US/firefox/29.0.1/releasenotes/

Thanks, I'm trying it.

But there are these drawbacks to this solution (all options on Privacy page):

1. Can't select "Always use private browsing", because it grays out the choices below it?

2. When I quit Firefox, because the solution says only to check Cache & Form/Search History on the Settings page, I can't have it auto. clear

    a. Browsing & download history (but will it anyway, unless I check it on the main Privacy page?)
    b. Active logins
    c. Cookies
    d. Saved passwords
    e. Site preferences
    f. Offline website data
               Am I correct about this?

Thanks, I did check the release notes, but only the third Fixed issue and the third unresolved issue are relevant, and I couldn't get any help from their detailed writeups. Did I miss anything?

When you run Firefox in Private Browsing mode then no data is saved to disk, so the is no need to clear any data.

What is the main cause of this issue is clearing the browsing history, so clearing other data shouldn't be a problem and you can easily test this.

You can let all cookies expire when you close Firefox and in that case there is no need to clear the cookies via "Clear history when Firefox closes".

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.

Clearing cookies will remove all specified (selected) cookies including cookies with an allow exception that you would like to keep.

Thanks again. So does this mean running it in Private Browsing mode should cause no problems, since no data (including browsing history) needs to be deleted ever? If so, why did I have the problem repeatedly when I ran in Private Browsing mode (that was always my standard mode for Firefox)?

If I don't run in Private Browsing mode, and don't check "Remember my download and browsing history" on the Privacy page, will that avoid the need to clear browsing history when I exit, since it won't be saved anyway?

By the way, what is "Offline Website Data?" Should I mark that for automatic clearing too?

Again, many thanks for all the help.

Offline Website data (see about:cache) is hardly used and Firefox asks for permission to store data if required.

See also "Tools > Page Info > Permissions" and the about:permissions page.

Not checking "Remember my download and browsing history" on the Privacy page should be sufficient to prevent Firefox from storing data.
Note that plugins may still store data in their own caches.
Plugins that obey Private Browsing mode shouldn't store data if you switch to Private Browsing mode and neither Firefox should store data.

What issues did you previously had in PB mode?

Note that data already stored in normal mode isn't affected and is saved to disk when you enter PB mode.

I had the exact same issue running in private browsing mode: Firefox left a process running after it closed, so I had to stop the process in order to reopen Firefox.

See also:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs