You mention several points. sending and/or displaying the wrong font or the wrong size font or randomly and invisibly changing font mid-message

What OS are you using? What version of Thunderbird are you using?

Are you talking about emails you are composing in a 'Write' window? Do you ever type into an MSWord document and then copy paste into a 'Write' composing window?

signature: Do you use a signature? Is this something you type each time or have you inserted the text into the 'signature text' box in Account Settings? OR did you create a html file and attach it? If html file, did you create it in Thunderbird write window or using something else like MSWord?

Thanks for the help Toad-Hall. I'm currently using Win XPsp3 and TB v38.3.0. I mainly use Reply-All and compose in TB (not Word). I often notice the problems in reply threads as they return. I have tried copying (the thread) out to Word but have been unable to completely undo the problems there and even found other problems (such as double lines) being introduced when I tried.

I have seen this problem occur in Replies you receive, if the sender responding uses an MSWord signature html file or copy pasted content from MSWord document that they composed. This adds a load of css coding that is not required and often effects how that email can appear to others including when you click on 'Reply' and try to type content.

If the sender to a response has sent email that has any hardcoded font size then this will also effect how things are viewed. This can also occur if they included a signature created in MSWord.

How to check to see if the email has inserted MSWord css coding. select email so you can see it in the Message Pane. in the headers area, click on 'More' and 'View source' or 'View' > 'Message Source'. The appended image below shows an example section of typical Microsoft coding in an email. If you see that sort of thing, it helps to explain what has occurred.

Settings to try in Thunderbird: They will be used as the default in the Display of received email content AND also when you compose an email.

via 'Menu Bar' 'Tools' > 'Options' > 'Display' > 'Formatting' tab or Via 'Mail Toolbar' 'Menu icon' > ' Options' > 'Options' > 'Display' > 'Formatting' tab

Under Fonts & Colours: Choose a suitable font you prefer eg: Arial click on the 'Size' drop down and select eg: 14 click on 'Advanced' button Make sure all the Size and Minimum font size are the same eg: 14

If you want to allow your received message to display fonts the sender selected: select: 'Allow messages to use other fonts' click on 'OK'

Then select 'Composition' > 'General' tab Uder HTML: Font: select 'Variable width' and Size: 'medium' This means it will use the font and size you had selected in the 'Display' settings and will not hard code the outgoing message so that it will display as per the recipients preferred settings. It is best to do this because you may not know what OS or what the available fonts are on the recipients computer.

Text colour: select a colour - this will be the default colour used in messages you compose.

click on OK.

If you set up default settings, the size and font and colour only appears that way for you in the Write composing window. It allows the recipient to select their own preferences like setting a larger size to read emails.

You can still use the 'Formatting Bar' to add hard coded content like making a word bold or italic etc. Anything you select on the Formatting Bar is inserted as html code to appear that way in any email you send.

Thankyou very much Toad-Hall for very clearly explaining this confusing area. I have adjusted my options exactly as you recommend and will monitor it for a while. I don't use signatures.

If I do copy paste content from MSWord, is there a way I can prevent it causing the problems you describe?.

And similarly if I receive an email which appears to contain css coding as you describe, is there a way I can amend it to prevent it causing problems in my reply?

Thankyou again.

I have found that when corresponding with outlook users, my "medium" font is coerced into a specific size in their replies, usually rather larger than I would choose. To combat this, I usually select all (ctrl+a) and apply a specific font size to the whole message just before sending. I use an addon which provides a font size setting button on the toolbar. IIRC, ExtraFormattingButtons.

If you need to paste from external sources, try Edit|Paste Without Formatting (ctrl+shift+v) .

This issue has been driving me bonkers for years. Both on brand-new messages and replies. One solution I have found is to install the add-in QuoteandComposeManager (0.4.0.2). If you can't find it using the 'search for add-ins', see https://freeshell.de/~kaosmos/quoteandcomposemanager-en.html

It's very strange that a simple add-in can add font stability that the base program cannot. I'm using v 42 beta on a Mac and keep hoping that the programmers will fix this someday.