In order for us to better investigate this issue you need to add a little more non-personal information.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?

Yes, the forward button is active in safe mode; I use AVG for my antivirus and firewall - this has never caused an issue in the past. The version is 17.5.3022 Here is the troubleshooting info:

Application Basics Name Thunderbird Version 52.2.1 User Agent Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 Profile Folder (Local drive) Application Build ID 20170622064432 Enabled Plugins about:plugins Build Configuration about:buildconfig Memory Use about:memory Profiles about:profiles Mail and News Accounts ID Incoming server Outgoing servers Name Connection security Authentication method Name Connection security Authentication method Default? account1 (pop3) pop.gmail.com:995 SSL passwordCleartext smtp.gmail.com:465 SSL OAuth2 true account2 (none) Local Folders plain passwordCleartext Crash Reports Report ID Submitted Extensions Name Version Enabled ID Lightning 5.4.2.1 false {e2fda1a4-762b-4020-b5ad-a41df1933103} Important Modified Preferences Name Value browser.cache.disk.capacity 358400 browser.cache.disk.filesystem_reported 1 browser.cache.disk.smart_size_cached_value 358400 browser.cache.disk.smart_size.first_run false browser.cache.disk.smart_size.use_old_max false dom.apps.reset-permissions true extensions.lastAppVersion 52.2.1 font.name.monospace.el Consolas font.name.monospace.x-cyrillic Consolas font.name.monospace.x-unicode Consolas font.name.monospace.x-western Consolas font.name.sans-serif.el Calibri font.name.sans-serif.x-cyrillic Calibri font.name.sans-serif.x-unicode Calibri font.name.sans-serif.x-western Calibri font.name.serif.el Cambria font.name.serif.x-cyrillic Cambria font.name.serif.x-unicode Cambria font.name.serif.x-western Cambria font.size.fixed.el 14 font.size.fixed.x-cyrillic 14 font.size.fixed.x-unicode 14 font.size.fixed.x-western 14 font.size.variable.el 17 font.size.variable.x-cyrillic 17 font.size.variable.x-unicode 17 font.size.variable.x-western 17 mail.openMessageBehavior.version 1 mail.winsearch.firstRunDone true mailnews.database.global.datastore.id 65d48007-e77a-467b-8956-45051399329 mailnews.database.global.views.conversation.columns {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false… media.gmp.storage.version.observed 1 network.cookie.prefsMigrated true network.predictor.cleaned-up true places.database.lastMaintenance 1501261999 places.history.expiration.transient_current_max_pages 122334 plugin.importedState true privacy.donottrackheader.enabled true security.sandbox.content.tempDirSuffix {6836cb85-7f7b-4bbe-9534-97ecddb802b3} Graphics GPU #1 Description Intel(R) HD Graphics 3000 Vendor ID 0x8086 Device ID 0x0126 RAM Unknown Drivers igdumd64 igd10umd64 igd10umd64 igdumd32 igd10umd32 igd10umd32 Driver Version 9.17.10.4459 Driver Date 5-19-2016 Features Direct2D false DirectWrite true (10.0.15063.413) WebGL Renderer false AzureCanvasBackend skia AzureCanvasAccelerated 0 AzureFallbackCanvasBackend cairo AzureContentBackend skia JavaScript Incremental GC 1 Accessibility Activated 0 Prevent Accessibility 0 Library Versions

Expected minimum version Version in use NSPR 4.13.1 4.13.1 NSS 3.28.5 3.28.5 NSS Util 3.28.5 3.28.5 NSS SSL 3.28.5 3.28.5 NSS S/MIME 3.28.5 3.28.5

Yes, this seems to have fixed the problem. Thanks to your team for all your help!