I am getting a repeating message that Thunderbird is trying to install a helper tool.

Please post a screenshot of the error.
https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

I'm not aware of any 'helper tool' for Thunderbird, so this may be a scam.

Thanks for answering! Attached is screenshot. A few times Thunderbird has started installing and asked me to restart Thunderbird.

Please post your Troubleshooting Information.
At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information.
Press the "Copy text to clipboard button" and paste the information into your reply.

Then try to start Thunderbird in safe mode.
https://support.mozilla.org/kb/safe-mode-thunderbird

Do you still get the 'Install Helper' pop-up?

Here it is. I will restart in safe mode now. Thanks.

 Application Basics

   Name: Thunderbird
   Version: 68.7.0
   Build ID: 20200407160932

   Update Channel: release
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:68.0) Gecko/20100101 Thunderbird/68.7.0
   OS: Darwin 15.6.0

   Multiprocess Windows: 0/0
             Disabled
   Remote Processes: 0
   Enterprise Policies: Inactive
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) mail.runbox.com:993, SSL, passwordCleartext
     OUTGOING: , mail.runbox.com:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Crash Reports for the Last 3 Days

 Remote Processes

   Type: Count

 Extensions

     Name
     Version
     Enabled
     ID

       Amazon.com
       1.1
       true
       amazondotcom@search.mozilla.org

       Bing
       1.0
       true
       bing@search.mozilla.org

       DuckDuckGo
       1.0
       true
       ddg@search.mozilla.org

       Emoji
       1.68.3
       true
       emoji@ganss.org

       Google
       1.0
       true
       google@search.mozilla.org

       Twitter
       1.0
       true
       twitter@search.mozilla.org

       Wikipedia (en)
       1.0
       true
       wikipedia@search.mozilla.org

       Lightning
       68.7.0
       false
       {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Security Software

   Type: Name

     Antivirus:
     Antispyware:
     Firewall:

 Graphics

     Features
     Compositing: Basic
     Asynchronous Pan/Zoom: none
     WebGL 1 Driver WSI Info: CGL
     WebGL 1 Driver Renderer: NVIDIA Corporation -- NVIDIA GeForce 9400M OpenGL Engine
     WebGL 1 Driver Version: 3.3 NVIDIA-10.0.52 310.90.10.05b46
     WebGL 1 Driver Extensions: GL_ARB_blend_func_extended GL_ARB_ES2_compatibility GL_ARB_explicit_attrib_location GL_ARB_instanced_arrays GL_ARB_internalformat_query GL_ARB_occlusion_query2 GL_ARB_sampler_objects GL_ARB_separate_shader_objects GL_ARB_shader_bit_encoding GL_ARB_shading_language_include GL_ARB_texture_rgb10_a2ui GL_ARB_texture_storage GL_ARB_texture_swizzle GL_ARB_timer_query GL_ARB_vertex_type_2_10_10_10_rev GL_ARB_viewport_array GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_depth_bounds_test GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_texture_compression_s3tc GL_EXT_texture_filter_anisotropic GL_EXT_texture_mirror_clamp GL_EXT_texture_sRGB_decode GL_APPLE_client_storage GL_APPLE_container_object_shareable GL_APPLE_flush_render GL_APPLE_object_purgeable GL_APPLE_rgb_422 GL_APPLE_row_bytes GL_APPLE_texture_range GL_ATI_texture_mirror_once GL_NV_texture_barrier
     WebGL 1 Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_disjoint_timer_query EXT_float_blend EXT_frag_depth EXT_shader_texture_lod EXT_sRGB EXT_texture_compression_rgtc EXT_texture_filter_anisotropic OES_element_index_uint OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
     WebGL 2 Driver WSI Info: -
     WebGL 2 Driver Renderer: WebGL creation failed:

• WebGL 2 requires support for the following features:

 transform_feedback2
     WebGL 2 Driver Version: -
     WebGL 2 Driver Extensions: -
     WebGL 2 Extensions: -
     Uses Tiling: true
     Uses Tiling (Content): true
     Off Main Thread Painting Enabled: true
     Off Main Thread Painting Worker Count: 1
     Target Frame Rate: 60

     GPU #1
     Active: Yes
     Vendor ID: 0x10de
     Device ID: 0x0863

   GPU #2

     Diagnostics
     AzureCanvasBackend: skia
     AzureContentBackend: skia
     AzureFallbackCanvasBackend: none
     TileHeight: 512
     TileWidth: 512

     Decision Log
     HW_COMPOSITING: disabled by user: Disabled by pref
     OPENGL_COMPOSITING: unavailable by default: Hardware compositing is disabled
     WEBRENDER: opt-in by default: WebRender is an opt-in featureunavailable-no-hw-compositing by runtime: Hardware compositing is disabled
     WEBRENDER_QUALIFIED: blacklisted by env: No qualified hardware

   Crash Guard Disabled Features

   Workarounds

   Failure Log

 Media

     Audio Backend: audiounit
     Max Channels: 2
     Preferred Sample Rate: 44100

     Output Devices

       Name
       Group
       Vendor
       State
       Preferred
       Format
       Channels
       Rate
       Latency

       Internal Speakers
       AppleHDAEngineOutput:8,0,1,2:0
       Apple Inc.
       Enabled
       All
       default: F32LE, support: S16LE S16BE F32LE F32BE
       2
       default: 44100, support: 44100 - 96000
       94 - 4176

     Input Devices

       Name
       Group
       Vendor
       State
       Preferred
       Format
       Channels
       Rate
       Latency

       Internal Microphone
       AppleHDAEngineInput:8,0,1,0:1
       Apple Inc.
       Enabled
       All
       default: F32LE, support: S16LE S16BE F32LE F32BE
       2
       default: 44100, support: 44100 - 96000
       30 - 2320

       Line In
       AppleHDAEngineInput:8,0,1,1:2
       Apple Inc.
       Enabled
       None
       default: F32LE, support: S16LE S16BE F32LE F32BE
       2
       default: 44100, support: 44100 - 96000
       30 - 2320

 Important Modified Preferences

   browser.cache.disk.amount_written: 768837
   browser.cache.disk.capacity: 1048576
   browser.cache.disk.filesystem_reported: 1
   extensions.lastAppVersion: 68.7.0
   font.internaluseonly.changed: true
   idle.lastDailyNotification: 1590260144
   media.gmp.storage.version.observed: 1
   network.predictor.cleaned-up: true
   places.database.lastMaintenance: 1589672627
   places.history.expiration.transient_current_max_pages: 112348
   security.sandbox.plugin.tempDirSuffix: 0be89757-63fd-bf4e-99d5-1d1fa28bd0ed
   signon.importedFromSqlite: true
   storage.vacuum.last.index: 1
   storage.vacuum.last.places.sqlite: 1589672627

 Important Locked Preferences

 Places Database

 JavaScript

 Incremental GC: true

 Accessibility

   Activated: false
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.21
     4.21

     NSS
     3.44.3
     3.44.3

     NSSSMIME
     3.44.3
     3.44.3

     NSSSSL
     3.44.3
     3.44.3

     NSSUTIL
     3.44.3
     3.44.3

 Sandbox

   Content Process Sandbox Level: 0
   Effective Content Process Sandbox Level: 1

 Internationalization & Localization

     Application Settings
     Requested Locales: ["en-US"]
     Available Locales: ["en-US"]
     App Locales: ["en-US"]
     Regional Preferences: ["en"]
     Default Locale: "en-US"

     Operating System
     System Locales: ["en"]
     Regional Preferences: ["en"]

It worked. Thank you very much!!

It worked.

Can you explain that in more detail?

HI, i followed the instructions you gave me above and when I opened Thunderbird after that the error message was no longer popping up. However just now I opened Thunderbird and the error message came again. I just closed the box and it was fine. I'll play with it today and if it continues I'll be in touch. Thank you very much.

Did the error message come back again?
Do you have any idea what could have caused this? Anything you did before this happened the first time?

The 'helper tool' window has appeared before with Firefox, and seems to be linked to a Mozilla app trying to update on a non-administrator account.

https://support.mozilla.org/bm/questions/1148314

https://discussions.apple.com/thread/251025270

https://apple.stackexchange.com/a/333265

The error message is gone now. Thanks to both of you!