Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but; some icons are locked in place.

There is also a Restore Defaults button at the bottom middle.

Note: about:customizing is no longer present in Firefox 47+ releases.

• Press Alt or F10 to bring up the toolbar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and
  select Customize.

That does not help at all. 1. about:customizing does not work, because I have 56.0.2 (64-bit) firefox. 2. Restore Defaults does not work at all. I've tryed it before. 3. I have that button, but it invisible. You may see screenshot by link http://prntscr.com/h632t1

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?

In safe mode everything is ok. But in normal mode with disabled plugins there is no button there.

There are two types of Add-ons. There are the Extensions that you find and install. And there are also Plugins. These are from other programs that are on your system when Firefox starts. Like video players, anti-virus, and many more. You can disable normal Add-ons by going to the Add-ons Manager,

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.

I've tried that already and that does not help. There is only one extention, that can't be disabled permanently - Kaspersky Protection. It's always on after restart.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Are you using the default theme or a custom theme? "3-bar" menu button or "Tools -> Add-ons -> Appearance" (about:addons)

Strange thing is that there is no + button in new firefox quantum

It's still invisible. http://prntscr.com/hj20l2

And there is no plugins because of bad programmers, who disabled support of old plugins(

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?

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox