It may be due to AVG antivirus, possibly disabling that might have helped the upgrade to Win 10. I fixed it by reverting to Win 7 and downloading Win10 so as to do a clean install on a different partition. (search Win 10 clean install) You can only reverse an upgrade to win 10 in the first month. They gave a questionaire why, top item was apps don't work, must be a common problem.

Firefox now works, haven't installed Thunderbird due to Windows 10 being designed to strip you of all your privacy - they have the right to anything you type while using it, can you believe it? Will be some time playing around with it, in no rush. My normal opinion is don't install a new windows version till the first service pack, they are giving it away to get us on it for the App store I think.

I think you might find your issue was the network adapter requiring an update. Something which has apparently been addressed in this weeks roll up patch to Windows 10.

This is a bug report, not a request for help on how to fix the problem myself. I don't have the skills, aptitude or inclination to fiddle with settings, nor do I know how to find out if the roll up patch to Windows 10 mentioned by Matt has been installed.

Firefox was buggy with Windows 10 but it seems to have been resolved because the problem I had hasn't shown up for a couple of days.

Thunderbird is buggy too. It has locked up a couple of times this past few days when I have several programs running and return to Thunderbird. . It's open, all the messages show in the inbox message list, but the only thing I could do this morning was click the red X to close the programme. I couldn't open messages, switch between mailboxes, download new messages, and couldn't get to a message that I was writing. The only solution seems to be to close T'bird and reopen it. That's a pain because I have to look for my partly completed outgoing messages in the draft folder again.

I don't know about technical stuff like network adapters and firewalls. I run Norton Internet Security. My internet connection is fine - Opera is working , so I am connected to the internet.

djp 2015-08-26

@DavidinDelta This is an Answer, there is nothing wrong with Firefox or Thunderbird to file a bug about. There is however an issue with Windows 10. It is Buggy.

So please contact the company responsible for the Bug and tell them what you think as we can not fix Windows for you. Nor can we fix Nortons Internet securities propensity to delete entire mailboxes and block Thunderbird from connecting to the internet.

As you apparently think you do not need to understand these things, and you do not, You had best take your device to a competent professional and pay them to service it, That is the price we all pay for not knowing how things work, be it a car, a water heater or a computer.

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Matt, no surprise, I thought I was posting to the Windows 10 forum.

Anyway, I'm more than happy with Firefox.

I'm less happy with Thunderbird, but it's nothing to do with Windows 10. The problem I have is that composing a message is a nightmare. Spellcheck flags parts of words as if they separate words, and the fonts change willy-nilly, almost always when I move text from one paragraph to another, but often just inexplicably. When I've received replies with my message included, I've been appalled at the mess Thunderbird creates.

This happened in Windows 8.1 as well as Windows 10. I don't recall if it happened in Windows XP Pro.

Half these problems could be solved if there was a way to control the font size the old fashioned way - setting a default for the message window, and having a box to explicitly say the font name and the font size.

Anyway, thank you for taking the time to reply.

My take on the fonts thing is press enter two or three time as soon as the editor opens and then return to the top of the message. Thunderbird has a habit of "going past" the end of formatting and putting those couple of additional returns helps enormously. The addon Quote and compose manager has an option which help some in stabilizing the font. See https://freeshell.de/~kaosmos/quoteandcomposemanager-en.html

Your suggestion re font size has one major flaw. HTML, which is what all "non" text messages are, scales size depending on the monitor. This is done for web pages and in email. So you specifying a size in point may actually see your mail scaled to the recipient as microscopic, a very common issue on this forum with Mac "Users as they traditionally have higher resolution monitors. or in the converse you message text may appear huge if you send it to someone still using a 600X800 display.

Pasting of information should be done using Ctrl+Shift+V to paste plain text, again it helps, although I regularly forget and make mu own mess. and press Ctrl+Z to undo and paste it again as plain text.

Another huge cause of font issues is signatures. People create them in Word and paste them into Thunderbird, or save the word document as HTML. Unfortunately having the entire contents of the Normal.dot file as an internal cascading style sheet is not very helpful if your not using Word.

The default font for the message windows is controls by; On the toolbar > Options > composition and On the toolbar > Options > Display

Note that where sizes are given with a numerical value it is in Pixels and these roughly convert to about 30% smaller that their points equivalent..

Finally is V38 as bad as previous versions. I seem to recall that there were some font bugs fixed in this release.