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

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

While still in Safe Mode,

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP in front of the numbers/letters, click it and select Submit.

Using your mouse, mark the most resent 7 - 10 crash reports, and copy them. Now go to the reply box below and paste them in.

Firefox may hang during or after downloading files if your download history has grown too large. To clear the download history:

1. Click the Download button icon (downward arrow in top right corner of browser) and then click Show All Downloads. The Library window will open.

2. Click Clear Downloads to delete the download history - this doesn't delete any products you have downloaded, it only clears the list of them.

3. Download some sample files to see if the hanging has stopped.

Also maybe check if your chosen download folder is still available. It may not be. Open it through your system explorer - I am not a Mac user and don't know what this is called on a Mac. In Microsoft it is called Windows Explorer. I am sure Mac has an equivalent.

4. Firefox may hang if the last download folder location (e.g. a shared volume or USB drive) is no longer available. To fix this:

• Click the menu button - the three bars in the top right hand corner of your Firefox browser - then choose Options.
• Select the General panel.
• In the Downloads pane, select the radio button beside Save files to and then click the Browse button.
• In the Browse window, choose a new folder from those shown. This will be the destination folder for all of your downloads.

5. Now close the General preferences page. Any changes you've made will automatically be saved.

See if you are now able to download files and/or save images without Firefox hanging. If this works, you may like to go back to your Firefox settings, open Options again, as you did above, and instead change the location by selecting Always ask me regarding where to save your downloaded files. This way you can choose different destination files for each separate download, rather than they all going to the same folder. it is up to you.

Even so, your download history can still become bloated, so may need clearing as instructed above, from time to time.

Did you try to reboot the computer?

See also:

• https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding
• http://kb.mozillazine.org/Profile_in_use