https://support.mozilla.org/en-US/kb/update-firefox-latest-version 49.0.2

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Thank you!

- re Update:  Apparently I am "up to date" even though I
have 47.0.1, not 49.0.2. Curious about that.
- * re Safe Mode: I  revisited a few sites I'd used recently, 
ones with dropdown menus that Firefox choked on, and 
tried using the drop downs.  Success!  
- re Regular Mode:  Your solution would be even better if
you gave a couple of URLs to pages with dropdown  menus
rather than asking users to remember all the pages
that Firefox hung up on.  I take notes on many things
all the time, but not on that.
I will be happy to come back to this page and click on 
SOLVED in a few days after I have successfully made
selections from a variety of dropdown menus.

Thanks again.

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

For now, the good news is that Firefox has stopped hanging up for dropdowns. So the safemode/restart trick (a) works and (b) makes further experimentation difficult.

If I encounter the problem again, I would be happy to do the experiment. It would help if YOU could provide a URL to a page that has a variety of drop down menus, some short and some long. (I say that because I observed that sometimes I could select from a single dropdown with a few options without hanging up Firefox.) That way I - and anyone else who wants to do this experiment - can use that webpage as the standard for identifying the evil plug-in.

By the way, for quite a while I have blocked adobe flash. When a page uses it, I get the request whether to continue blocking and I ALWAYS say YES. Even reinstalling and updating adobe flash, which I did quite a bit there for a while, did not help. (The adobe flash problem is separate from the drop-down menu problem. The latter persists despite always blocking adobe flash.) Blocking adobe flash means that I have to use other browsers for a few things. Google calendar, for one. Watching Hamilton's America, for another. Edge does fine with both.

Thanks again!

Enable Flash, then install this. No more "request whether to continue blocking"

Flash block {web link} Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on it

Dear FredMD: Alas, the latest Firefox also hangs up. (50.0.2). Is there a way to make the fix that I do myself permanent throughout all future Firefox versions? Or can Firefox make the necessary changes in its own code, or at least automate the fix so it runs when you install the latest version (after asking if you want it)? Thanks.

I will call for more help for you. But,

A post was flagged as Solved Problem. Please remove that flag. If you don’t, others that can help may not read this post.

Go to that message, and use the Undo link to its right.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

I did that last time (see top of thread) and it worked. What I'm asking this time is whether (a) Mozilla is capable of fixing this bug once and for all given that it persists in each new update, and (b) if the fix is too hard, whether Mozilla could find someone to write a macro (or anyway, that's what we all used to call it) so that each time an update leads to the problem coming back, at the end of installing the update, the user could be asked if she wants to run that macro to solve the hang-up-on-drop-downs problem. This user would always answer YES. For now, when I get the hang up, I start Edge and use the URL that hung Firefox up. That's quicker for me. As I recall, doing your fix took 10+ minutes. Doing mine takes less than a minute each time. And by the time I've done it 10 times - if not sooner, there's a new Firefox update so I get a new 10 minutes to play with by NOT doing your fix. You are probably not the right person to ask/tell but maybe you know who is. Thanks! Roberta

Hi Roberta, could you clarify the problem for me? It sounds as though it's a form control where you click it and a list drops down below and you click an item to select it and then move on to the next form control. Is that where the problem is?

(1) Does the page freeze up on you so you can't do anything, or is the problem just with that part of the form (i.e., the list won't open, or it opens and it's blank, or it's got items but clicking them doesn't do anything)? I'd love to get a better picture of the problem.

(2) Is it an HTML form or a Flash/Silverlight/Java form? Usually if you right-click an area that is part of a plugin-controlled form, you'll get a short menu mentioning the name of the plugin instead of Firefox's usual right-click menu.

Not sure if the error will occur on this simple demo page: https://jsfiddle.net/q8cf1rub/

FredMcD said

I will call for more help for you. But, A post was flagged as Solved Problem. Please remove that flag. If you don’t, others that can help may not read this post. Go to that message, and use the Undo link to its right.

DONE!

jscher2000 said

Hi Roberta, could you clarify the problem for me? It sounds as though it's a form control where you click it and a list drops down below and you click an item to select it and then move on to the next form control. Is that where the problem is? (1) Does the page freeze up on you so you can't do anything, or is the problem just with that part of the form (i.e., the list won't open, or it opens and it's blank, or it's got items but clicking them doesn't do anything)? I'd love to get a better picture of the problem. (2) Is it an HTML form or a Flash/Silverlight/Java form? Usually if you right-click an area that is part of a plugin-controlled form, you'll get a short menu mentioning the name of the plugin instead of Firefox's usual right-click menu. Not sure if the error will occur on this simple demo page: https://jsfiddle.net/q8cf1rub/

Thanks, jscher2000.

Here are the answers: 1. The page freezes so I can't do anything. This does not happen with short drop-downs (I think, but haven't done a careful experiment) but does with longer ones, or subsequent ones if there are several of those. What happens is that I get the 'waiting' circle and then the top banner says "Firefox (not responding)".

Sometimes I can just x the window and then start Firefox again. But sometimes I have to use the Task Manager to end task.

I can send you a picture if you send me a sample with some long drop-downs. I tried your link and had no problem. However, that page did not appear as a normal web page but was in edit/compose mode. That might make no difference, but it seemed like it was not the most clinical way to do the experiment.

2. It's an HTML problem. I don't get the name of any plug-ins.

If you can send me some regular web pages with a series of dropdowns of varying length and complexity - not necessarily ones you have composed but ordinary ones - I can evaluate them. Also, the next time I have the problem on a page of general interest, I will take a screen shot.

Thank you!